Interpreting capability statistics
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Cp, Cpk, Pp, and Ppk statistics are measures of potential and overall capability. Because the process information is reduced to a single number, you can use capability statistics to compare the capability of one process to another. Many practitioners consider 1.33 to be a minimum acceptable value for the process capability statistics. A value less than 1 indicates that your process variation is wider than the specification spread. To understand and interpret the capability statistics, use the information below for guidance.

Capability statistics allows you to monitor and report the improvement of the system over time. The capability statistics differ in interpretation depending on the distribution used for the analysis. For,

Normal distribution:

·    Cp and Cpk - Cp does not consider the location of the process mean in relation to the specification limits. Cpk is an index which measures how close a process is running to its specification limits in relation to the process spread. Cpk is higher only when you are meeting the target (or process mean when no target value is specified) with minimum variation. For example, a process may be performing within minimum variation but away from the target (or process mean) and closer to one of the specification limits, will result in low Cpk and high Cp.

·    Pp and Ppk - Pp and Ppk are interpreted in the same way as Cp and Cpk respectively.

·    Cpk vs Ppk - Minitab calculates Ppk using the overall variation Both the between-subgroup and within-subgroup contributes to the overall variation. Cpk is calculated using the within-subgroup variation, but not the shift and drift between subgroups. Ppk is for the whole process. If Cpk and Ppk are the same, then the overall standard deviation approximately equals the within-subgroup standard deviation.

·    Cpm - Minitab calculates Cpm when you specify a target value. Use Cpm to assess if the process is centered at target. The higher the Cpm index, the better the process. If Cpm, Ppk, and Pp are the same, then the process mean coincides with the target.

Nonnormal distribution:

Minitab calculates the capability statistics using 0.13th, 50th, and 99.87th percentiles for the distribution used in the analysis. The distance between the 99.87th and 0.13th percentiles is equivalent to the 6s spread in the normal case. The 50th percentile represents the process median.

The capability indices based on a nonnormal distribution can be interpreted in the same manner as the indices based on a normal distribution. Compare Pp and Ppk to assess if the process median is close to the specification midpoint. If Pp is greater than Ppk, the process median is off the specification midpoint and closer to one of the specification limits.