Leaderboard

Benchmark Six Sigma Expert View by Venugopal R Hypothesis testing is no doubt a very powerful method for objectively deciding whether we have enough reason to believe that two populations are different. Once we understand the concept of hypothesis testing, one can discover that it has potential to be applied in almost all the phases on DMAIC. However, if we need to look at some of the key reasons why the tool is not patronized to the extent it could be, I would put down the below points, though these may not be exhaustive. 1. A green belt professional can gain adequate proficiency and confidence in the use of TOH only by repeated practice and deep thinking. The few examples used in a GB training are meant to illustrate the tool and its application, but many more examples need to be tried out. 2. From the various examples that are done, the participant needs to relate situations in his / her work area where the type of data used can be comparable. For instance, an example from a manufacturing situation can be compared to one in a services industry as far as the data is concerned. It could be “number of units produced vs number transactions served”. 3. The non-availability of statistical software like Minitab, Sigma Magic or equivalent has been seen as a deterrent. Most participants get trained using a trial version and later they are not equipped with the software. 4. Many a time, the leader (& sponsor) is anxious to implement improvement actions and do not spend adequate time and effort to have baseline data. Once improvement is done, even if they want to compare with the ‘before’ situation, they are constrained due to lack of baseline data. 5. Participants are sometimes unsure of the choice of the tests as applicable to their projects. Hence, they tend to avoid using this tool, in fear of using a wrong test. 6. The sponsors and other senior management leaders may not have the knowledge to appreciate the usage of Test of Hypothesis, which could discourage the GB to try it out, unless strongly supported by a good Blackbelt / Master Black belt. 7. The ability to interpret the results in a “Business language” rather than a “Statistical language” is another important skill for a Project leader to impress the benefits derived by using TOH, and other tools. 8. There may be some instances where the volume of data available could be very large, or the delta is large, to show very obvious differences between populations, which could render the usage of hypothesis testing as redundant. 9. There could be some who would not have gained an acceptance nor belief to the usage of the method and continue to be comfortable with ‘gut feeling’ decisions. There would be many other reasons as well, which I expect other ambassadors to narrate. On the whole, usage of TOH will be improved with more mentor-ship, exercises, making the software available, and getting the senior leadership exposed to appreciate the use and power of such tools.

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.