Section V.F.4 of the CSSBB body of knowledge says to calculate the process capability for attributes data. To accomplish this, The CSSBB Handbook published by the ASQ says to convert the percent non-conforming to a Z-value and divide that by three to get the process capability. But, The ASQ CSSBB Study Guide says that the centerline that would be used for the attributes control chart is the equivalent process capability. To throw another perspective into the mix, Montgomery's Introduction to Statistical Quality Control 7e only has a discussion on PPM measures under the header of process capability for attributes data. Montgomery's solution seems to be the simplest and most universal in my experience, but I can also understand that there may be situations where using the Z-value divided by three could be useful in some contexts. The idea of using the centerline from the control chart as a measure of process capability does not make any sense to me, though.
What thoughts do you have? What have you seen used as process capability measures for attributes data and how were they calculated?