SPC Calculation Style

The SPC Calculation Style determines the method used to compute capability indices during analysis, as well as when the application's feature for calculating control limits is used. This also defines which control limit trend rules are applied. Capability indices and control limit calculations can be based on one of the following methods:

• Average and Range computations
  • Sigma = r/d2
  • Where d2 is an industry standard constant value published by the AIAG.
• Average and Standard Deviation computations
  • Sigma = s/c4
  • Where c4 is an industry standard constant value published by the AIAG.
• Uses mathematical routines to compute the actual standard deviation computed for only the measured data.

The first two methods of calculation style estimate the sigma values for the data population by using measurements from sampled subgroups. Both methods assume that the underlying data follows a normal distribution.

The standard deviation method offers a more accurate calculation by directly calculating the standard deviation from measured data. However, most calculations are based on estimates.