There are various rules of thumb for equating Cp & Cpk to Sigma scores, such as 1.33 Cp = 4 Sigma(short therm). If you can calculate Cpk, then you can calculate the Z score (assuming normal distribtuion). Cpk is the minimum of [(USL – X(bar))/(3*StDev)] and (X(bar)-LSL)/(3*StDev).
If you have all the details to fill those formulas out, you can calculate the % yield using the Z distribution and then convert that to a Z-score. If you have MINITAB go Calc>Probability Distributions>Normal Distribution. Put your average and standard deviation in, do a cumulative probabilty, and put your LSL in for the constant. Write down the result. Do the same for the USL. Subtract the two (this is your % Yield). Then go back to Calc>Probability Distributions>Normal with the mean of 0 and standard deviation of 1. Put your % yield (calculated before) in the constant field. This is the Z score (sigma level).
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