These calculators compute the process capability index
which shows the process potential of meeting the specifications. Enter
the process parameters and specifications in one of the following
tables, depending on whether you have a double-sided or single-sided
specification.
Historical note: "Six sigma" really started with Cpk...
Motorola wanted to tolerate process shifts (i.e., changes in the process
mean) of up to 1.5σ in either direction but still wanted to ensure a low
defect rate. If the USL and LSL are both set to be 6σ (six sigma!) away
from the mean, then a maximal process shift of 1.5σ would still produce
the magical number of 3.4 defective parts per millions (PPM).
