Ppk, Process Performance, often referred to as Long term capability, the overall capability indices are associated with the overall sample standard deviation. The calculation of Cpk in MiniTab takes in to account the effect of time ordering by using the observed level of variation taken from a Moving Range, Range or Sigma control charts. These indices reflect how the process could perform relative to the specification limits, if the shifts and drifts between subgroups could be eliminated. The MiniTab definitions are: But note some texts have the definitions reversed.Ĭpk, Process Capability, often referred to as Short term capability, the potential (within) capability indices are associated with the within-subgroup standard deviation. This is a common problem, your Excel calculations are calculating Process Performance Ppk not Cpk I noticed Excel, because of the formula used, will not use RBar/d2 to estimate sigma nor take subgrouping into consideration. I used a subgroup size of 2 on Minitab. Knowing the formulas used for calculating all Minitab methods would hepl. I real thought of myself as a SPC guru until now. Please help! I now don’t know which Cpk to report. Please see formulas used on Excel and data below. I then tried all the other different methods (RBar, SBar, Pooled, and Biased/Unbiased) that Minitab provides of estimating sigma and neither one resulted in the same Cpk as Excel.

condition and low Cpk in graphical format using Minitabs Process Capability Six Pack. Although, the Cpk reading (-2.17) I got using Excel showed up on my Six Pack as a Ppk (used Pooled/use an unbiased constant section unselected). I was about to send the supplier the data indicating the out-of-spec. I just ran a capability study using Excel (I always used Minitab).

Process capability on excel