It is good to start "Six Sigma Way" with the qustion "Why Six Sigma?". First of all what is sigma?.... Sigma is the standard deviation. Standard deviaton on the selected the data pattern. Data Pattern of what?.. Data pattern that interests analysis. Let us assume for a while that our depeartment prodcues bolts of 10 cm daimeter. There are two ways of achieving this goal. First approach is with having too much dependency on the person producing the products. Second approach is to have formalized process guidelines in palce to build the bolts. I think, we are matured enough to understand that we want to be process oriented while building our products. Okay... If process is what builds the products.. then it is our responsibility to understand the capability of our process.. How do we decide this?.. It is only possible with the products (i.e. bolts) produced by our process. Thus, we want to analyze the data pattern to check the process adherence thus want to have an opinion on process capability.
Okay.. Now I understand why we need to focus on data pattern. Why we are selecting stand deviation (i.e sigma) intead of mean, mode etc.. Mean and mode can not represent process capability if we have positive and negative variation from the expected bolt daiameter. If we have two bolts with 9.8 cm and two bolts with 10.02 cm. What it is not possible to continue analysis on "zero" mean. Without getting into statistical depth, let us belive in stasticians for a moment. Statisticians say when we normalize the data and plot the graph, we can see a interesting curve pattern around the mean.
Lower specification and Upper specification limits specy what variation customer can afford to. For example for bolts company customers specified limites can be 9.70 cm and 10.30 cm. These form the Lower Specified and Upper Specified limits. We need to measure these Specification limits distance from the mean and can take the help of below statistical table to calculate current process capability index.
Thus, we can conclude for now that the more sigma our specification limits are from the mean, the less defects our process produces. Thus, we can take sigma way as metric to improve our process performance. That is we can decide what is the sigma number our current process capability and from there we can take going to 3 sigma, 4 sigma, 5 sigma and 6 sigma as our goal to improve our process.
Thus, Friends, Sigma way is an approach to calculate process competence. If customer are confident about our process competence, they can be rest assured on the products produced because of it. It is similar to person with human values and good character is most likely be good in future also....