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Vishwadeep Khatri

Process Stability, Process Capability

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Differentiate between a stable process and a capable process. Is Process Stability supposed to be a pre-requisite for all type of processes? Explain with appropriate examples. 

 

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Process Stability refers to the consistency of the process to stay within Upper Control Limit (UCL) and Lower Control Limit (LCL). The outputs of any process will have a mean value. Then the control Limits for the process are defined as follows:

  • Upper Control Limit (UCL) = Mean + 3 sigma
  • Lower Control Limit (LCL) = Mean - 3 sigma

If the process behaves consistently over time, i.e. the outputs fall within the range UCL to LCL called process width, then the process is said to be stable or in control.

If the Outputs are spread across outside the limits, then the process is Unstable or Out of Control.

 

Process Capability is a measure of the ability of the process to meet customer specifications. The measure tells how good each individual output is. An estimation of the ppm (defective parts per million) is a method to measure process capability.

Capability Indices (Cp, Cpk) are metrics used to measure the process capability. It indicates how capable the process is in terms of meeting customer requirements.

Customers provide Upper Specification Limit (USL) and Lower Specification Limit (LSL) within which they want the product value to exist. This is called the tolerance or Allowed variation. (E.g. A customer of a Building Management system may want his air conditioning such that room temperature is 20 +/- 2 (deg C) i.e. with USL =24 deg C and LSL = 18 deg C)

The air-conditioning process may have its own Mean/Average temperature say, 21 deg. C

  • Cp = Tolerance/ Process Width = (USL-LSL) / (UCL-LCL)
  • Cpk = min(Cpu,Cpl) where Cpu measures the closeness between process mean and USL; Cpl measures the closeness between process mean and LSL. Cpk accounts for change is process mean.

A simplistic example: In an exam if the passing mark is 40 (USL = not specified; LSL = 40) and a student if 5 continuous attempts get 30,30.5,31,32,30 then we can say he is Stable(consistent) but not Capable.

 

Is Process Stability supposed to be a pre-requisite for all type of processes?

 

Any manufacturing /production process should be stable before being released to the production environment. Many customers request that their suppliers submit process capability data to qualify the supplier process. Any estimate of process capability depends entirely on where the process happens to be when the data is collected. For an unstable process, the mean shifts about over time. So, stability would be prerequisite for such process, as the process capability can only be estimated for stables processes.

 

But, I believe, processes involved in Research and Development, Innovation would be inherently unstable.  These process may not need a Capability estimation , so stability will not be a prerequisite.

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Stable process is one where the variations are predictable and the causes of the variations are also predictable and are common causes inherent within the system. Basically all possible special cause of variations in the system are understood and addressed. Capable process are one where process is primarily stable and in addition it is good enough to meet the specified goals consistently. In case of stable process distribution of data will be consistent over period of time.

Process stability has to be pre-requisite for all processes, though stable process does not mean process is capable and vice versa also is true. But capability of process is the state at any point of time, whereas stability is the state of process over period of time. And also assessment of capability is against a specified goal, its important to make sure that process is understood well before declaring it capable. In that context its important to confirm stability before assessing capability.

Example would be a insurance claim handling process where time taken to close the claims would be stable means that the process is consistent in handling time with known variations. Unless its confirmed to be stable how do we make sure that process is robust and capable to meet customer demand. It may happen that some claims breach the SLA due to unusual behavior of the process. So for a proactive monitoring of process stability is primary reference followed by capability.

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Stable process works within specified control limits and without excessive variation. It means that the process will produce stable output at all times and it would be easy to predict how well the process would perform in future and produces consistent results over time. It shows how capable the process is in meeting the customer needs time and again. Instability is the result of variability in process. For e.g. if the equipment is not maintained regularly it breaks down. Stability is not only a requirement for flow, but helps in developing flow to disciplined approach to stability. A stable process is a prerequisite for calculating capability because a process that’s out of control is not a process. 

 

Capable Process is a process that has achieved its specified quality & product goals and objectives. It is the ability of the process to produce output that meets specifications .A process is capable if nearly 100% of the output from the process is within the specifications. A process can be in control, yet fail to meet specification requirements. In this situation, some steps needs to be taken to improve or redesign the process. If there are no points beyond the control limits, the process is said to be in statistical control. . A control chart analysis is used to determine whether the process is "in statistical control" and involves only common cause variation and not special cause variation. The elimination of special cause variation does not improve a process.  All it does is get the process back to where it should have been all along. Control charts are important because they can indicate that a process is stable

 

Process stability and process capability are different ideas and there is no relationship between them. Knowing that the process is capable (or not capable) will not tell anything about process stability. And knowing if the process is stable (or not) will not tell anything about the process capability. Finding out the stability of a process will help determine whether there is a problem and help identify the source of the problem. Knowledge on the stability of the process is also a prerequisite for further analysis, such as capability analysis. Before assessing process capability, process should be stable. An unstable process is unpredictable. If process is stable, future performance can be predicted and improved. So at all times, the word stable needs to come before the word capable.

 

Thus meeting targets of customers and being within specification limits is very important.

Changing engine oil after every 2000 miles is advisable as also is putting the right amount of oil in the car. Not too little or not too much of oil else malfunctioning of car is possible.

A shirt size of 40 may be required but one can instruct that size between 39 and 42 is acceptable, though preference would be for Size 40 only.

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A "stable" process is one in which there are no special causes of variation. The entire variation present in the process is contributed by common causes and no special patterns are observed if one were to produce a control chart. Some patterns which indicate an unstable process are: a single point outside 3 sigma, 2 out of 3 successive points beyond 2 sigma on the same side, 4 out of 5 successive points beyond 1 sigma on the same side, 8 successive points on the same side of mean etc. A stable process would be indicated by the absence of any of these patterns.

 

On the other hand, a "capable" process is one which is consistently able to produce outputs that meet specifications. i.e. the UCL and LCL of the process should be within the USL and LSL provided.

Generally, stability is considered as a pre-requisite since a stable process can be predicted. A process that is unstable cannot be predicted, i.e. it could be currently producing conforming products but at any time, it could begin to produce non-conforming products. Thus an unstable process cannot be safely assumed to be capable, even if it currently seems so. The only occasion when stability might not be much of an issue is when the margin of safety is very large. i.e. the specifications are wide enough to accommodate even the wildest swings in performance.

Consider the example of a pizza delivery joint having strict delivery time SLAs. The USL for preparing the pizza is 20 mins so that the delivery boys can comfortably deliver it to the customer within 45 mins.

 

Assume that the pizza making process is unstable and though it usually takes between 12 to 15 mins to make a pizza, sometimes it has taken upto 22 mins due to some problems with the oven. In this case, the process is good enough (capable) most of the times but it cannot be safely assumed to be capable at all times due to the inherent instability. The owner of the joint should try to fix the problem with the oven so that the process is first made stable.

However, if the delivery SLA is relaxed to 1 hour, the same unstable process might be acceptable, as the delivery boys will still have enough time to do the delivery even if the pizza takes 22 mins to produce.

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Differentiate between a stable process and a capable process. Is Process Stability supposed to be a pre-requisite for all type of processes? Explain with appropriate examples. 

Sr.no

 

Stable process

 

Capable process

 

Definition

 

A process which is able to produce the output consistently over a long period of time

A process which is able to produce the output meeting customer specification

Pre-requisites

 

Customer specification (USL/target/LSL) is not required.

Customer specification (USL/target/LSL) is required.

Variations

 

It has only common cause of variation.

It can have special cause of variation along with common causes.

Output predictability over a long period of time

 

We can predict the output over a long period of time

Since there are special causes of variation, it will be slightly difficult to predict the output over a long period of time

 

Before going to example, I would like to clarify that a stable process does not mean that it is a capable process and vice versa.

 

Definitely process stability is a pre-requisite for all the type of process.

 

Case study:-

 

A company is producing a product which has a weight of 3Kg.

 

Stable process:-

 

Bellows distribution curve shows that the process is stable, as it is making product consistently at a mean of about 3.002 Kg.

image.png.137fdb42fdd374a9fb35497cc9bffaa2.png

 

Non stable process:-

 

Now over a period of time if there is some special cause of variation, than the same process cannot be stable process. We can get a graph as shown below.

 

image.png.4ae807a1e88a03b8ec9f6f8c8844eba3.png

 

We can see from the graph that there are two spikes in the distribution curve, one is near 3000 and another one is around 3025. At the mean value 3012.43 we are getting very few readings.

 Product should be of 3 Kg, but due to some special cause entire mean after that cause has shifted to 3025.

Thus this process is not stable.

Capable and stable process:-

 

Now in the same process, when we add customer requirements, we can see whether the process is capable or not. From the below distribution curve we can see that we are getting weight of the product well within specification.

This process is both stable and capable.

image.png.ffbff121c89ff4ed707fb7ecd3d20089.png

 

 

Capable process but not a stable process:-

 

      

image.png.a63ae6687951c4d8899e088a7710ff4a.png

In above distribution curve of weights, we can see that the product is well within specification, but it is not stable. It is very difficult to predict the output of the process.

Also minimum value is very near to the LSL. Now if customer revised the specification limits and makes it more stringent. In that case our process will not be capable to meet the requirements. And we may lose the business.

Hence it is very important the process is stable. Once we eliminate special causes of variation from the process and make it stable. It becomes easier to adopt new customer requirements. The only thing required will be to shift the mean towards target.

 

As an example if the customer comes up and tells us that he requires a 2.5 kg of product instead of 3 kg.

We will have to change the process mean to 2.5 kg and the process will be capable on its own. As we know that the process is stable.

 

 

 

 

 

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Two stages are to be attained for a process to be in a statistically controlled state of certainty and quality. Being stable and capable are the requirements for a setup to maintain consistency and conformance to the customers’ requirement.

STABILITY – is an ability of a process to perform in a predictable manner over a period of time. The process in this state will be aware of the common cause inherent in the dynamics. Random causes noticed during Stability drive will be analyzed for the root cause and mistake proofed. The goal during stability period is to contain the output within control limits (set by the manufacturer/provider).

 

Control charts provide an insight to the voice of the process.

 

CAPABILITY – is an ability of a process to realize the characteristic that will fulfill the customer requirement. The process in this state will have to be contained within the specification limits (decreed by consumer) agreed with the customer. Assignable causes are made remote if not banished in the process.

 

Capability indices are calculated using the specification width divided by process spread. Indices which determine the spread, the location and long term capabilities are helpful to assess the performance of the process. Ideally a process spread with the output has to be close to the mean of the controls and centered to the specifics.

 

Capability is effectively conceived only if the process is religiously stable. Below are the situation an organization/process can be in.

 

Is Stability achieved

Is Capability achieved

Status

Risk

Solution

Examples

NO

NO

Both control limits and specification limits are breached.

Type I error of considering common cause as special cause is possible as the reviewer would try to tweak the system for reduction in variability.

Taming special causes, acknowledging common causes and deriving  control limits

New employee’s yield/QC score to the company. Stage of assignable causes fed back as a learning. The production from the employee has to be at par with the department’s variation with the output.

YES

NO

Control limits are honored yet overshooting by the specification limits

The customer not happy with the output

Specification limits are checked for business compatibility. Consistently adhering within the control limits is another option.

New application launched for an android. Bugs autocorrected yet not meeting customer expectations with frequent crashes of the system.

NO

YES

All of the output is within the specification limits but out of control.

Type II error of special causes treated as common cause. Mean shift has to be addressed.

The process needs to be controlled. Special causes out of the control limits should be analyzed. Process to be reviewed.

Wait time due to the poor categorization of the IVR. Customers waiting for equal time for resolution irrespective of the severity of the issue. Missed callers are reached out by email. CSAT varies as stability is missing.

YES

YES

The outputs are within the control

and specifics

Complacency and latency with the processes. Stability to be under check.

Continuous improvement. Process reengineering

Aircrafts landing and takeoff instances at an airport. Timelines, communications and SOPs are deployed for error free service.

 

 

 

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Stable Process and Capable Process

In the Chapter entitled, “Common Causes and Special Causes of Improvement. Stable System” of his treatise, “Out of the Crisis”, Deming says, “As we shall learn, a process has a capability only if it is stable”.

  • A process that operates with its control limits is a stable process while one that operates within Specification Limits is capable.
  • For a process to be deemed as capable, it needs to be consistently capable. For the process to be consistent, it needs to be stable.
  • While process stability and process capability are not related, the key connection is that Process capability assessments should be performed after demonstrating stability of the process.
  • Process capability assesses ability to meet specifications. But, with an unstable process, it is difficult to assess or predict its capability. With an unstable process, the estimate of the process capability becomes relevant only at that point of time.
  • The capability of a stable process can be improved, but an unstable process cannot be considered capable.

In conclusion, while stability and capability need to be treated together in terms of conclusions about the process, it is imperative that “stable” comes before “capable”.

 

Process Stability – A pre-requisite for every process?

As mentioned above, stability of a process confirms that its capability can be predicted better. Stability is a characteristic that is observed over time. In other words, is a process as good now, as it was before and will it be as good later? This assumes a certain repetitiveness of the process within a reasonable time-frame.

 

But for processes  which have  a long time interval between them or when processes are more of one-off or once-in-a-long-time or even once-in-a-lifetime event, in which there is nothing like predictability because there is no definite future for the process or no likelihood of the process happening again in the conceivable future. In such cases, process stability, although important as a concept may not be quite relevant.

 

Examples of such processes can include construction projects, large machine assemblies, equipment erections, rocket-launches, Software upgrades, ERP implementation, Functions, Various Financial, Process or System audits and so on. In these “processes”, process stability may not be a prerequisite.

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Definition of A Capable Process is a process that has achieved it's specified quality and product goals and objectives.

Definition of  A Stable process is a process  that is operating within specified control limits and without excessive variation.

There is no link between the two- They both attribute 2 different condition or status of a process. Both are very important aspects in the manufacturing processes.

An example to elaborate on the difference between a capable process and a Stable process, that I can think of by experience is a Goods delivery Van that delivers raw food stuff to a QSR on a daily basis.

 The Process is Food Delivery.

The Specification : The timing is specified as delivery touch at the outlet at 6 am in the morning.

Process Flow: The Van follows the Google map to find the fastest route and the truck is loaded at 4 m every morning as the average driving time from the start point to the destination is 120 minutes.

Actual scenario: For 2 years, the van driver has been taking multiple routes with different (Best) traffic times with the objective to reach by 6 am. Unfortunately, for some reason or the other he has not reached before 6 am. While the delivery process results has not been capable (The delivery time is always beyond spec- approximately  in the range of 10- 15 minutes ), it has been pretty much consistent- consistently beyond Spec time. The outlet staff however always knew what to expect (approx.by 6:10- 6:15 am), so we could always consider that though not capable the process was at least stable and within a manageable delay time. So we can say that the delivery process is stable but not capable.

Data Points : 730 Days with varying delivery times- as can be imagined the spread is big

Day

Spec Time(Hours)

Arrival time

Deviation

1

6:00:00

6:05:15

0:05:15

2

6:00:00

6:09:15

0:09:15

3

6:00:00

6:11:11

0:11:11

 

It is obvious that that process stability is supposed to be the prerequisite for all processes to predict it’s capability. To define or assess the process capability, we must know or assess the ability of the process to meet specifications. So, we cannot access the capability of the process if the process is not stable.

There are several methods to measure Process capability – Indices such as Cp, CpK, Pp, Ppk are common. Trying to summarise the capability of a process through only one indice is not advisable, however, only to demonstrate that the assessment of the capability of a process needs to be done only when the stability of the process is confirmed, let us consider the example provided above

In the above example :

The process capability index Cp represents the acceptable tolerance interval spread in relation to the actual spread of the data when the data follows a normal distribution. The Formula is: 

 

Cp = USL-LSL

                                                                        6s

 

WHERE

USL and LSL are the upper specification limit and lower specification limit, respectively, and 6s describes the range or spread of the process.

The range or spread of an unstable process, , will not be conducive to calculate or arrive at a decisive Capability index. The estimate of process capability depends on how and where the process happens when we collect the data. So if  the process average is shifting about over time, the measure of process capability will be invalid.  The estimate of the Cp only reflective of where the process is at the point of time when the data is collected and not where it may go next. The above process is stable and hence the average of the data point spread is dependable. If the delivery timings had to be haphazard and the range of variations had to be big- the capability of the process would have been indecisive.

Hence Process capability Assessment should only be done after the process is stable.

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Process stability

A process is considered stable if its output is within predictable limits. The most popular method used for assessing process stability, when its output is a variable data, is by studying the frequency distribution of the output and ensuring that it follows a normal distribution.

 

For example, the commuting time from my home to my office varies between 25 to 35 minutes, with an average time of 30 minutes. If I collect the data for a period of 100 days and plot a frequency diagram and see if I get a normal distribution, I can consider this commuting process as stable and predictable. The normality can also be tested statistically.

 

Stability and Capability

However, if I set an expectation that I have to always maintain a commuting time of less than 30 minutes, the above process, though it is stable, is not capable to meet my expectation.

 

A car which is giving a consistent mileage for fuel consumption may be considered as a stable process, since it is predictable within a range, but need not be considered capable with respect to a certain expectation.

 

Thus, it is possible to determine the stability of a process by using the output data generated by the process itself. However, the capability of the process is always by comparing the operating range of the process output with respect to certain specified expectation.

 

The process capability could be impacted either due to a “mean shift” or due to “Excess variation” or both.

 

When the overall variation of the process is less than the limits (USL – LSL), then the process potential is good. However, only if the process is centered in such a way that the variation is contained well within the specification limits, the process capability is considered as good.

 

The range reference that is used for calculations pertaining to process potential and process capability is 3 standard deviations on either side of the mean.

 

Illustration of Process Potential & Capability

For example, the room housing a complex medical equipment requires a temperature setting between 16 and 18 degrees. However the data based on the existing air-conditioning equipment gives a range of 15 to 16 degrees. Here, since the range (2 degrees) is maintained, we may believe the process has the potential to meet the requirement, however the setting can to be altered to bring it within the required range to make it capable.

 

On the other hand if the existing equipment’s performance varies from 14 to 17 degrees, we cannot bring the process within the required range by changing the settings. Then the process does not even have the potential to meet the requirement

image.png.c0bde6ea8c82c7dfcfd6d34de868e5d3.png

 

Process stability – is it a pre-requisite always?

Process stability is always desirable, however it need not be a pre-requisite in all cases. Let us examine the following cases with examples:

 

Spec limits wide enough to accommodate

Suppose we have process whose overall variation is very low compared to the specification limits. The non-normality or the mean shifts would classify the process as statistically in-stable, however, it may not have a practical adverse effect. Eg. pneumatic clamps for holding a job in position. The air output from the compressor may not be stable due to the cut-off settings, but as long as the minimum clamp pressure is attained, it is acceptable.

 

Robust designs

Another example is the way many of the modern electrical appliances are built to work under a wide range of input voltage variations. Hence, even if the input voltage fluctuations do not constitute an output of a stable process, the equipment design is robust to work with those instabilities.

 

Storage and release methods

Where we depend upon certain natural inputs, which are subjected to spikes of variations challenging their process stability, we have little or no control. In such cases we no longer expect the stability of these inputs as a pre-requisite, but design our recipient processes suitably.

Examples:

·       Wind is an instable input that is made use of by Windmills and subjected to storage systems that help in generating useful energy.

·       Natural water resources need not be stable but are consumed by a hydroelectric station to provide consistent power.

·       Varying Solar energy is harnessed by solar panel and batteries to ensure useful supply of electricity

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Stability of a process and capability of a process both are entirely different things. Stability of a process means the process is statistically under control i.e. there is no assignable cause of variation and the observed variation is entirely due to chance causes. Stability of a process is assessed with respect to control limits which are based on Natural Limits of a process derived from 3 sigma (standard deviation) limits of normal distribution of the process. Though these limits are not calculated directly as in control charts (say X bar R chart) we plot X bar values and not individual data and hence these control limits make use of Central Limit Theorem. If the X bar values are with in control limits and don't exhibit any set pattern, the process is said to be stable.

Capable process is always with respect to the Specifications Limits, its basically assessed by comparing span of Specification Limits with the span of Natural Limits (which are nothing but +/- 3 sigma limits). In other words a capable process is one which has Cp i.e. process capability more than 1, preferably more than 1.33. For a six sigma process, process capability should be 2.

 

Cp = (USL - LSL) / 6 Sigma

 

Yes, process stability is a prerequisite for all types of processes. A capable process should be essentially stable first. Stability of a process is entirely a statistical property, it has nothing to do with the specification limits of the product being produced through the process. 

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A capable process can be stable process but a stable process may or may not be a capable process. Every process has some inherent variations called common cause variation, we can not ignore that . Special cause variation is other that common cause which is more that +- 3 sigma. A process is said to capable if it comes under process curve between LSL & USL. A process is said to be stable if it is +-3 sigma around its mean value, which means we are getting value within 6 sigma. whether a process is capable or not, measured through process capability indices. Cp is Specification limit divided by process width but it does not tell us where the process is laying, where its mean or it is shifted. we need Cpk because if mean shifts still Cp value will remain same. If Cp is not equal to Cpk it means that process has shifted . Process mean tells us which side process has been shifted LSL or USL. 

Shifting the mean is very easy rather than to reduce variance. Special cause can be manageable.

 

A process stability may be supposed to be prerequisite for all type of processes.Process stability refers to the consistency of the process with respect to mean, variance or other important process characteristics. A process is stable because it has consistent value around its mean. The process distribution remains same over a period of time. we can find out whether process is stable or not by Xbar - R Chart, X bar- Chart, Moving range chart.Process capability is an assessment of the ability to meet specification limits. If the process is unstable, we can not predict its capability. For example if process mean has been shifted and if process is stable then only we can predict from Cp or Cpk that where it is going towards LSL or USl but if it is not stable we can not predict Process capability. 

For example time to reach office in morning is 8:45 am to 9 am. A person is coming to office, his timing recorded over a period of time and it was found that he comes in between 8:55 am to 9:10 am. in one way he is stable while coming to office within that time span of 15min and consistently coming on that but it is out of specified time. So this process of coming to office is stable but not capable because it is shifted towards USL and he will be most of the. So by finding process capability we can find out where the process is shifted and work on that but for that process should be stable.

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Process Capability and Process Stability:

Process Capability is the ability of the process to perform or to produce the output with in the control limits. This is known as VOP

Process Stability is the consistency of the process with respect to its set parameters. This is referred with respect to its average value. 

Difference between a Stable Process & a Capable Process:

Stable process refers to consistency in the output. If the variation within the process outputs is less, this is more stable process.

Whereas a capable process is more towards the process being able to maintain its output values with in the specification limits.

A stable process can give us better performance level. 

A capable process can give us the process to remain within the limits. 

 Is the stability in the process is a prerequisite for a capable process?

It is prerequisite yes. 

If we see closely, they both are very much connected. 

For example:- if a machine produces a shaft with inconsistent diameter, obviously there are high chances that the output variable or values go out of control leads to incapable process.

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Stable Process

A simpler definition : A process which can be consistent over a period of time, in producing its output is a stable process

 

Capable Process : A process which can meet the target mean and customer specification limits . Process capability can be measured by two terms , Cp & Cpk. Cp talks about Process Capability and Cpk talks about process performance.

 

Cp  is a ratio of tolerance of width to the short term spread of the process. It does not consider the centre of the process. Cp assumes that the process is stable

-          If Cp < 1, process is incapable.

-          If Cp=1, then this process meets the expectation barely , there could be defects atleast .3 %

-          If Cp > 1, then this process output falls within specification, but defects could be there if the process is not centred .

-          If Cp=2 , then we achieve 6 sigma

 

Cpk on the other hand, considers process centring. It is a ratio of the distance measured between the process mean and the specification/tolerance limit closer to half of the total process spread. It assumes that the process is stable

-          If Cpk=Cp, the process mean is on target

-          If Cpk=0 then the process mean falls on one of the specification limits, 50% of the process output falls beyond the specification limits

-          If Cpk<-1 the process mean falls beyond both specification limits and therefore 100% of the process output is out of specification limits

 

A process is assumed to be statistically stable before we calculate its capability. So process stability is of paramount importance for all types of processes, especially if we are running a DMAIC project. But if we are not able to control the existing process and are unable to make it stable or if the improvements that we make on the existing process do not yield the necessary or expected results, then its better to go for a new process through DMADV or DMAODV

 

Now as we have seen what stable and capable process are all about, let us see the impact of a process being unstable.

 

What could be the reasons for a process to be unstable

1.      It could be because of special cause variations of different nature. Common cause variations are inherent to the process and hence cannot be a cause

2.      It could be because the design of the process may not be good enough to meet the customer expectations.

 

Eg:1

 

Let us see how special causes can impact stability

 

Everyday , the local agent for Aavin milk, needs to provide milk to all of his customers between 5 AM – 5:15 AM IST.  He chooses a pre-defined route on a routine basis and goes by the same route daily and delivers milk, by a bicycle. But every now and then, he misses the timelines by 10-15 minutes and delivers late ranging from 5.25 AM to 5.30 AM IST.

 

This is because , one day he sees road blockage in few lanes due to plumbing work done for water connection. As a result, he has to take a circuitous route and then on an another day, sees a road blockage because of telephone work happening. On some other days, it could be because the milk delivered to the agent was delayed and in a different occasion, because stray animals were there in the street, he has to take a different route and this delayed his delivery timelines. 

 

As we can see here, these are all different reasons and all of them special causes which make this process of - delivering the milk, unstable. So we need to find an alternative way of improving this process (DMADV or DMAODV). One way is the agent having a motorbike to improve his speed of delivery. Other option is to find better or closest alternate routes for each of the problematic lanes/streets. First option is much better though

 

Conclusion:  All processes need to be stable in general.  A capable process is assumed to be statistically stable as well. When an improvement is being done for an existing process which requires stability, and if we are not able to see the  necessary yield or stability in the process, then we go for a DMADV or DMAODV project.

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S.No

Stable process

Capable process

1.

Process operating within specification limits

Process operating with in control limits.

2.

Variation is more

Variation is less

3.

Process is within specification limits

Process is in statistical control

4.

It meets customer needs

It meets customer and business needs

5.

First step to go for process improvement

If process is stable then only process capability should be performed.

6.

All stable process are not capable processes

But all capable processes are stable processes

7.

No prerequisites for stable process

Pre-requisite of the process capability analysis is a stable process

 

 

Is Process Stability supposed to be a pre-requisite for all type of processes?

 

Process stability is required for all the quantitative data of all types of processes. In a process, every parameter or every item which gives quantitative data will have specification parameters in place. So the process should perform stable before going to capability of process.

Process capability assessment should only be performed after first demonstrating process stability.

 

 

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Stable Process : A process which works within a "predictable range" of Performance Parameters. These processes usually are either seasoned and matured, or developed to perfection or have minimal possibility of Variation. The focus here is on "Stability".

Such processes need minimum monitoring

 

Capable Process : A process which can deliver the "Output" inspite of any possible variation in parameters that can affect the Outcome. In this case, the focus is on "Customer". It is capable as long as the rejection / defect rate is well within the prescribed standards.

Such processes relatively need more attention than a Stable process.

 

Is Stability Mandatory?

Yes, that's the Wish. However, in a fast evolving world, processes are allowed or are cleared to be used before the "Best" Stability is attained. For Example, in the much publicized Galaxy Note 7 Fiasco which costed Samsung in Billions, the processes that were to validate the health of the battery in usage conditions might not have achieved the "Stability" standards. But, considering the tough timelines, the product was manufactured & released. It could also be that some factors that affect the "Stability" might have been missed : The X's !!

 

That said, business would always love to have a Stable Process, thought in some situations they may not have a choice but to adopt a Unstable processes and strengthen the later checks to ensure desired "Excellence" Standards !!

 

The Galaxy Note 7 Fiasco Detail :

http://www.firstpost.com/tech/news-analysis/the-samsung-galaxy-note-7-fiasco-a-blow-by-blow-account-3695113.html

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A process is said to be stable when the variations are always within Upper and Lower Control limits whereas a process is said to be capable when the outputs of the process meet the specifications or the customer requirements.

 

In a Stable process, the Special cause variations would be absent. Only Common cause variations would be affecting the process. In such we may not be sure whether the process is throwing up desired outputs which can meet the client requirements. Therefore we cannot be sure whether it is a capable process. 

 

In case the process is capable and not stable, meaning that it may be producing results which do meet desired specifications, however instability of the process may at times throw up results which do not meet the specifications. Hence Stability of a process is extremely important before we can certify a process to be capable process.

 

For example, a train is required to take about 60 mins to run from Station A to Station B as per the railway timetable. So 60 mins is the specified time. However, due to some reason or the other if the train consistently reaches its destination within 90 mins +_ 5 mins then the process would be stable but not capable since it does reach within the specified time of 60 mins and therefore does not meet customer requirement. Now consider another scenario wherein the train reaches it destination at times within 60 mins however, many times it would reach in 60 +- 15 mins. In such condition the process may be capable but not stable. It still does not meet customer requirement consistently. Hence process stability is a pre-requisite to process capability.

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STABLE PROCESS AND CAPABLE PROCESS

Differentiate between a stable process and a capable process. Is Process Stability supposed to be a pre-requisite for all type of processes? Explain with appropriate examples…

 

What is Process Stability― Is the variation obtained when the same person measures the same unit with the same equipment over an extended period of time. It is used for checking Data Homogeneity (Special causes are present or not).

It can also be explained as the ability of the process to perform in a predictable manner over time, and the data is collected in a random manner. If the sample data is not stable, it would mean that the data is coming from 2 or more different processes. In this case, it is useless for any prediction about the population.

The following things are checked for stability using RUN CHARTS

  • Trends: Trend are sustained and systematic source of variation characterized by a group of points that drift either up or down. Trend warns that a process is about to go out of control.

  • Oscillations: Oscillation is when the data fluctuates up and down rapidly, indicating that the process is not steady.

  • Mixtures: Mixture is characterized by an absence of points near the center line or mean. Mixture often indicates combination of two populations, or two processes operating at different level.

  • Clusters: Cluster may indicate variation due to special causes, such as measurement problem, or sampling from bad group of parts. Cluster are group of pints in one area of the chart.

Thumb Rule: If the p-value for all the above is greater than “0.05” process is considered stable

i.e.

Ho = Data is Stable

Ha = Data is not stable

So, if the p-value for all the above is greater than “0.05”, then you fail to reject null hypothesis

 

What is Process capability ― Is the ability of a process to produce required output within specification limits specified by customer. This is the overall capability at which the process is operating. Process capability is measured by Z value. More the Z value is better the process capability.

The above definition give lead to two terminologies i.e.

  • Upper Specification Limit (USL): A value that is specified by customer and represents the highest range of a variable. The upper specification limit is the benchmark below which a product or service performs. In addition, the upper specification limit is often accompanied by a lower specification limit, but not always, as many products will have only one control limit.

  • Lower Specification Limit (LSL): A value that is specified by customer and represents the lowest range of a variable. The lower specification limit is the benchmark above which a product or service performs.

     

Who to calculate process capability:

To calculate the process capability, we need to calculate “Z” value. “Z” value is calculated as per the below chart

                           
      Data Type      
           
                           
                           
  Continuous Data         Discrete Data  
           
                           
                           
Normal Data     Non Normal Data     Binomial     Poisson
                           
Normal capability Analysis to find Z Value     Respective capability analysis to find Z value     Binomial capability analysis to find Z value     Poisson capability analysis to find Z value

 

Difference between Process Stability and Process Capability

Process Stability

Process capability

Is the variation obtained when the same person measures the same unit with the same equipment over an extended period of time

Is the ability of a process to produce required output within specification limits specified by customer

We check "Trends", " Oscillation", "Mixture" and " Cluster" to see whether process is stable or not

We check "Z" value to see whether process is capable or not

Limit specified in stability check is process upper control limit and process lower control limit

Limit specified in capability check is customer upper specification limit and customer lower specification limit

upper control limit and process lower control limit is coming from process performance

upper specification limit and lower specification limit is coming from customer requirement

 

Answer to the question “Is Process Stability supposed to be a pre-requisite for all type of processes”

Yes – even before proceeding with further analysis of identifying the potential cause (i.e. “x”) for the identified effect (i.e. “Y”), one should bring the process to stability by addressing the reason for non-stability or out layers.

Once the out layers are addressed and if you see that the process is capable, then you need to take the buy in of stakeholder to whether to pursue the project or to stop there

If you see that the process is not capable, then proceed with your project to bring the process within the capability.

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Most people got this right. In summary stability is seen over a period of time and capability is calculated at a point in time. In our quest to create the excellence glossary, I am biased towards someone who gives a concise and well written answer covering all aspects. In this case, I will choose Arunesh's answer as the best highlighting key points & important definitions
 

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Question: Differentiate between a stable process and a capable process. Is Process Stability supposed to be a pre-requisite for all type of processes? Explain with appropriate examples. 

Process stability: means consistently producing the output in the process over time. If a process is consistent over time & distribution of the data is within the control limits. then we call this process as “Stable within control”. If there is a shift from the mean but the data points fall into the control limits set is called “Stable – out of control”. This requires attention since there is a common cause variation.

 image.png.dfd866e8b81c5a9394c1d51e78b34f4d.pngimage.png.d4874e5a01275d90c08626aa4cfe99de.png

Process Capability – means a measure of the process and how capable the process is in maintaining the customer’s needs / expectations. It explains us how good or how bad the output is. Process capability is measured and represented in “Cp, Cpk, Pp, Ppk”.

If the process is stable but not capable, the customers will still be satisfied but not pleased/delighted.

Eg. A customer requires pant size of 40 for his usage. Also he says anything that fits him between 38 & 42 would be okay. In this case, the store consistently supplies him the size of 39. The customer is happy but not delighted. When the supply is accurate the customer would be delighted. Here the process is stable but not capable in meeting the customer’s expectation. You should always concentrate on a target to delight the customer and not on the range though given by them.

 

Process stability

Process Capability

Definition

Predictable output consistently.
How well a process behaves to produce the output in future. Stability has nothing to do with capability

Meet customer's expectation at all times.(100%) It compares the process performance agaings tehe specifications given by client. Capability has nothing to do with stability. Both has inherent relationship.

Central tendancy measures

Constant mean and constant variance is required to say a process is stable.

Range and target is provided by the client.

Limits

Control limits(UCL & LCL ) are in place

Spec limits (USL & LSL) are in place.

Usage of variations

Constant random variations / controlled variation is exhibited by a stable process

Controlled and uncontrolled variations can be seen in the process.

Grphical tool used

Control chart / Scatter plot

Histogram

Interpretation

A random fluctuation around the constant mean over a period of time is said to be the stable process. When the pattern is seen and variation is uncontrolled, though it falls within the control limits, the process is not stable. It is always important that we require controlled variation, and constant mean.

if the histogram falls within the specification limits, the process is capable. If the variation is too high when mean is shifted below the target, then the process is incapable.

Distribution

Constant distribution is required to say the process is stable.

Normal distribution is required to say process is capable.

Order of stability / Capability

Stability comes first

only when stability and normality of the process is tested, a process capability is tested.

Measurement index

Root cause for variation would be identified.

Cp, Cpk, Pp and Ppk is calculated.

 

Usage of Cp & Cpk, Pp & ppk

Use Cp, Cpk for samples and Pp & Ppk for population to arrive at the capability index.

Definitions
 Cp= Process Capability. A measure for a centered process
 Cpk= Process Capability Index. Adjustment of Cp for the off centered process.
 Pp= Process Performance. A measure of process performance for the centered process.
 Ppk= Process Performance Index. Adjustment of Pp for the off centered process.

Cp – Capability index will tell you how fit the data are into the USL & LSL. Also make sure the process is well centered around the average.

If the process is centered – then Cp is equal to Cpk. If not centered, then Cp > Cpk.

When the Cp & Cpk is > or = 1 the nthe process is capable.

Cp & Cpk comparison to sigma

Cp and Cpk of:

·         1.00 is equal to 3 Sigma

·         1.33 is equal to 4 Sigma

·         1.67 is equal to 5 Sigma

·         2.00 is equal to 6 Sigma

 

. Is Process Stability supposed to be a pre-requisite for all type of processes?

Deming has quoted” Only when the process is stable, the process is capable of producing output”. This means the capability cant be checked before studying the stability and normality. The order is like this… Study stability, normality and then capability of any given process.

A stable process is always a prerequisite for all processes to meet customer’s expectation or calculating Process capability, because a process can’t be capable if the process is out of control.

In a DMAIC process , a BB should always check

1.       Cehck if the process is stable or not simply by using the control charts.

2.       Check if the data is normal or non normal to calculate the capability.

If the process is not stable , then start focusing into the root cause. Try to eliminate the roort causes identified from the control chart, make the process stable. Make sure the data is normal. Then calculate the capability. Remember if the process is out of control, then no use in calculating capability. Because capability depends on the data where the process happened when the data is collected.

Always remember root cause identified is eliminated ,but not to improve the process, to get the process where it belongs to.  Dr. Deming used to use a very simple analogy in his seminars: “If this building catches on fire, we must put out the fire.  Putting out the fire does not improve the building.  All it does is get the building back to where it should have been all along – no fires!”

For Eg. When we apply for the credit card, the agent tells us that we would receive the kit within 7 working days. But here customer expects a shorter delivery. Though there are aware of the process, they expect the shortest. Anything that exceeds the time period mentioned by the agent, will annoy the customer. There the process is said to be unstable and the variation would be  high. In such cases, the customer makes many phone calls to the agent ./ bank to check the status. This involves cost, time and human intervention. Also, this makes the business weaker sinceword of mouth is the best method to increase sales pitch in the bank. It ruins the sales of the company.

 

This variation is to be identified and eliminated inorder to satisfy the customer, and put back the process where it had been / where it should be to improve this process.

 

Conclusion:

 

Then process has to be stable , the data is to be normal and then the process is calculated whether it is capable to meet the customer needs.

 

Any process which is unstable can’t be capable to meet the customer’s expectation.

 

In simple words thus, stability and capability need to be treated hand in hand in terms of interpretations, but at all times, the word stable needs to come before saying the word capable.

 

thanks

Kavitha

 

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Process Stability Capability
to measure the resultant of the process is repeatable and consistent measure process ability whether it meet specifications
Process distribution is consitent over the time indicates how good inputs are
  Capability cannot be performed if process is not stable
A Stable process is one that is operating within specified control limits and without excessive variation A Capable Process is a process that has achieved it's specified quality and product goals and objectives

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