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Control Chart

 

Control Chart is a graphical tool used to determine process performance with time. These were developed by Walter A. Shewhart and hence are also known as Shewhart charts. These charts help determine whether the process is stable or not. If the process is stable, the process will only have inherent variation (Common Cause variation). The region of common cause variation is given by Control limits (Upper and Lower control limits). Any process variation outside these limits is considered as Special Cause Variation.

 

Pre-Control Chart

 

Pre-Control Chart is a tool used to quickly check the status of a process. It is a tool used to actively manage a process and is more commonly used for high capability processes. It divides the specification limits into 2 zones - Green and Yellow. Green zone is exactly in the middle of the Specification width. The boundaries of Green zone determines the Pro-Control Limits. Area outside the specification limits is considered as Red zone. Samples are picked and checked as to which zone they fall in and depending on which zone the units fall in, one of the following decisions is taken

1. Continue running the process
2. Stop the process and fix it

Pre-Control Chart is typically used by shop floor workers / machinists to determine if some action is required on the process or not.

 

 

An application oriented question on the topic along with responses can be seen below. The best answer was provided by Bheemannadora Sappa on 13th September 2018. 

 

 

Question

Q. 92  What are pre-control charts and how are they different from control charts? Highlight the construction, advantages, disadvantages and applications of the same.

 

 

Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.

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Pre-control charts are the charts made using USL and LSL limits. it is normally preferred VOC. the tolerance between these USL and LSL is divided into two zones, Green and Yellow, Out of tolerance is considered as Red zone.

 

Green is 50% of the tolerance in the center, Yellow is 25%+25% of tolerance to both sides of green.

If the part specification falls either in Green Zone or Yellow Zone, it is acceptable.

If the part specification falls in Red Zone, it is not acceptable.

Pre-control chart with the above limits are printed in paper with color bands and do charting in shop floor.

It helps shop floor operator to control the process so that defective parts are not produced.

 

There are defined rules to qualify the process / setup.

 

How Pre-control charts different from control charts, Advantages:

1. Control chart preferred voice of process, but pre-control charts preferred VOC.

2. Control charts are difficult to understand by shop floor operators, whereas Pre-control charts are simple to understand by shop floor persons.

3. Control charts need sample data and the control limits are derived from the data, whereas Pre-control charts the control limits are Green Zone.

4. Control charts doesn't give the behavior of the process from very beginning, whereas Pre-control charts gives the process behaviour from very beginning.

5. Control charts needs subgroup data to define the control limits, whereas Pre-control charts we are measuring individual measurements.

 

Disadvantages:

1. Pre-control does not concern itself with understanding or estimating process characteristics vital to achieving product quality.

2. Any obvious patterns over time that indicate trends or cycles are not easily detected.

3. Since Pre-control charts uses individual values, it provides little power to detect anything but large process changes. So, pre control chart cannot help detect process change and variation unless drastic.

4. Pre-control is reactionary in nature and does not focus on finding or eliminating the sources of process changes. It leaves it up to the operators to make adjustments to stay within specifications and gives very little attention to eliminating variation at source.

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Construction: Pre Control charts are constructed keeping specifications on the Y-axis and the number of events/days on X-axis, with a central zone of 2 S.D limits, a peripheral zone of 3rd S.D, and a red zone of Medical Intolerance..

We can plot daily patient lab values on the chart to observe a general variability pattern, which remains constant in the stable population served.

 

Advantages:

1. Blessings for the service industry
2. No  financial expense on commercial controls
3. Pertains to the local population
4. Any instance of increase in variability will alert us, before the control values, giving us enough time to investigate the process error and set corrective steps, before the error actually reaches the patient investigation.

 

Disadvantages
1. Needs a statistically skilled black belt
2. Loads of work for  BB
3. The convincing ability of MBB
4.No pre-existing guidelines or format


Good Use:
Healthcare industry, particularly hospitals l

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We have limited responses today but I want to applaud both the Ambassadors for attempting this question which is based on a topic not known to all. This is what underlines the intent of this forum - we get to learn what we do not already know.

 

The chosen best answer today is that of Bheemannadora Sappa for covering all aspects, for the structure and lucidity of the answer. A call out to Arvind for attempting in the extended time!

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