Make Valid Control Chart and Subgroup Assumptions

Calculating Process Capability Without Control Charting
The third example occurred at an organization that was regularly reporting the process capability of business key processes at management meetings. On studying the methods used for the calculation it was seen that the process variables were never plotted on control charts. In addition, the process capability was calculated using the sample standard deviation formula.

What was wrong with the application described above?

In the calculation of process capability indexes, the first assumption that is critical is that the process is normal. By not validating this assumption the organization was running a huge risk of misinterpreting the voice of the process.

The company was also using an incorrect method of calculating the process capability. In the calculation of Cp and Cpk, the most important consideration is the use of standard deviation that is estimated as the population standard deviation - free of special causes estimated from the subgroups formed. The sample standard deviation calculated by the company included variation due to special causes, which is more appropriate for the calculation of Pp or Ppk.

In the process of simple process capability calculations, assumptions that were overlooked and the absence of control charts deprived the organization of important process knowledge that would help in understanding process variation. Understanding variation is the first step in continuously improving business processes.

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