I'm looking for a way to calculate what the comparable CP numbers are for various levels of a sample size.

If my target at 30 samples for example is a CP of 1.33 but I can't obtain the 30 samples and can only obtain 15 or even 20. How would I work out what the equivalent CP figure would be? Obviously I know it would need to be higher given the lower sample size but I don't know how to work it out.

Could any of you help with this please?

thanks in advance