MEASURING CAPABILITY

Just as under normal SPC, control must be established before capability is calculated; so it is with the short-run SPC. Once control of the process is established, process capability is estimated for either the short or long term . There are two ways to do this. The first one is to follow the general guidelines of the normal capability, provided all the data are coded and the calculations are based on the coded data. The second is to make sure that the estimated capability is based on individual part number, because the average, standard deviation, and specification limits vary for each part.

Whereas the calculations for the first method are exactly the same as those that we already have discussed in Chapter 12, in the second method, the calculations are much more demanding and beyond the scope of this book. The reader is encouraged to see Constable and Hobbs (1992) and Little and Harrelson (1993) for more information.

In essence, the capability for short runs may be summarized as follows : The short-term capability must be much greater than that for the long term and, in effect, becomes very optimistic. This is so because of limited samples and the lack of information of shifts and drifts in the process average.