Bias is calculated as the difference between the known standard value of a reference part and the observed average measurement.
When the actual measurement is unknown, Attribute
Gage Study (Analytic Method)
estimates the bias as follows:
- Point estimate of bias with a lower tolerance limit
- bias = lower limit + intercept / slope
- Point estimate of bias with an upper tolerance limit
- bias = upper limit + intercept / slope
The intercept and slope for both formulas are from the fitted line on the probability plot.
Minitab regresses the z–score Φ-1(Prob (Acceptance)) on reference values XT to calculate the intercept and slope.