A capability index is a ratio that relates the ability of a process to produce product that meets specification limits. The index relates estimates of the mean and standard deviation of the quality characteristic to the specification limits. Within estimates of capability are based on an estimate of the standard deviation constructed from within-subgroup variation. Overall estimates of capability use an estimate of standard deviation constructed from all of the process data. See Capability Indices for Normal Distributions and Variation Statistics.
Estimates of the mean or standard deviation are well-defined only if the processes related to centering or spread are stable. Therefore, interpretation of within capability indices requires that process spread is stable. Interpretation of overall capability indices requires that both process centering and spread are stable.
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When confidence intervals are not provided (for example, for nonnormal distributions) you can use the Simulate feature to construct confidence intervals. For an example, see Simulation of Confidence Limits for a Nonnormal Process Ppk.
When confidence intervals are not provided (for example, for nonnormal distributions) you can use the Simulate feature to construct confidence intervals. For an example, see Simulation of Confidence Limits for a Nonnormal Process Ppk.Guidelines for values of capability indices can be found in Montgomery (2013). The minimum recommended value is 1.33. Six Sigma initiatives aim for much higher capability levels that correspond to extremely low rates of defective parts per million.
For the nonnormal methods, estimates are constructed using two approaches: the ISO/Quantile method (Percentiles) and the Bothe/Z-scores method (Z-Score). For details about these methods, see Capability Indices for Nonnormal Distributions: Percentile and Z-Score Methods.
Most capability indices in the Process Capability platform can be computed based on estimates of the overall (long-term) variation and the within-subgroup (short-term) variation. If the process is stable, these two measures of variation should yield similar results since the overall and within subgroup variation should be similar. The normalized box plots and summary tables can be calculated using either the overall or the within-subgroup variation. See Additional Examples of the Process Capability Platform for examples of capability indices computed for stable and unstable processes.
The Process Capability platform provides two sets of capability indices. See Capability Indices for Normal Distributions for details about the calculation of the capability indices.