QUESTION
How is Cpk calculated for Single-Sided features such as flatness?
ANSWER
QC-CALC uses Cpku for Cpk for all single-sided upper tolerance features including Flatness, True Position, Concentricity, Parallelism, Perpendicularity, and Roundness.
QC-CALC does not calculate one side or the other on Single-Sided features. For example, a flatness only has a plus tolerance so QC-CALC calculates and uses the Cpku for Cpk. QC-CALC uses the only Cpk that is calculated for the particular side in use.
All calculations and reports in QC-CALC assume that your data set is a "sample" of the total parts, and not the entire set of data. For this reason, we always calculate "Sigma by N-1". This formula corresponds to Excel’s "STDEV()" function.
There are 3 methods of calculating Standard Deviation, also known as Sigma.
- σ by N-1 (Sigma)
- σ by S (Sigma_by_S)
- σ by R (Sigma_by_R)
Cpku based on upper tolerance = 
CpkL based on lower tolerance = 
Cpk = Lesser { Cpku, CpkL }