In order to answer your question so that you understand why the answer we give is the correct one we must first understand the concepts of Cpk and Ppk.
The first concept is that Cpk and Ppk formulas were originally designed for ongoing processes and was conceived as an extension of SPC. Properly designed SPC charts are constructed using a series of small samples – or subgroups – taken over time. In general, when properly designed the parts within a subgroup are created under relatively the same conditions: same raw material, same operator, same equipment, same settings, same tool/fixture/equipment condition, same environmental conditions, etc. Each subgroup is taken far enough apart that some and eventually all of these conditions change so that all variation is captured.
Within subgroup variation therefore represents short term variation. Cpk uses the within subgroup standard deviation (the average of all of the subgroup Standard deviations)
The total variation (within subgroup plus subgroup to subgroup) is the long term variation. Ppk uses the standard deviation of all of the individual values.
(It is important to note that often people use the notation of Cpk as a generic term for a process capability index. They will specify Cpk when they really mean Ppk.)
When a product – or process – is new, there is often limited variation in the various inputs. So there is no way to determine the long term performance. Short term capability can be assessed. The units are all to be created using essentially the same conditions: same raw material, same operator, same equipment, same settings, same tool/fixture/equipment condition, same environmental conditions, etc. Only the sample size is much larger than one would take for an ongoing sampling of multiple subgroups. Typically, the sample size would be from 30-100 units. In this case since there aren’t multiple subgroups the standard deviation is just the single standard deviation on the individual values. The same standard deviation formula as for Ppk. So the short capability calculation for new products looks like the Ppk formula…
Hope this helps
Note: there are many complicating factors that can influence the sampling and interpretation depending on your actual situation…