A process capability analysis compares the distribution of output from an in-control process to its specification limits to determine the consistency with which the specifications can be met. The CAPABILITY procedure provides the following:
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process capability indices, such as
and 
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descriptive statistics based on moments, including skewness and kurtosis. Other descriptive information provided includes quantiles or percentiles (such as the median), frequency tables, and details on extreme values.
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histograms and comparative histograms. Optionally, these can be superimposed with specification limits, fitted probability density curves for various distributions, and kernel density estimates.
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cumulative distribution function plots (cdf plots). Optionally, these can be superimposed with specification limits and probability distribution curves for various distributions.
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quantile-quantile plots (Q-Q plots), probability plots, and probability-probability plots (P-P plots). These plots facilitate the comparison of a data distribution with various theoretical distributions. Optionally, Q-Q plots and probability plots can be superimposed with specification limits.
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goodness-of-fit tests for a variety of distributions including the normal. The assumption of normality is critical to the interpretation of capability indices.
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statistical intervals (prediction, tolerance, and confidence intervals) for a normal population
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the ability to produce plots either as traditional graphics, ODS Graphics output, or legacy line printer plots. Traditional graphics can be saved, replayed, and annotated.
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the ability to inset summary statistics and capability indices in graphical output
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the ability to analyze data sets with a frequency variable
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the ability to read specification limits from a data set
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the ability to create output data sets containing summary statistics, capability indices, histogram intervals, parameters of fitted curves, and statistical intervals
You can use the PROC CAPABILITY statement, together with the VAR and SPEC statements, to compute summary statistics and process capability indices. See Getting Started: CAPABILITY Procedure for introductory examples. In addition, you can use the statements summarized in Table 5.1 to request plots and specialized analyses:
Table 5.1: Statements for Plots and Specialized Analyses
|
Statement |
Result |
|---|---|
|
cumulative distribution function plot |
|
|
comparative histogram |
|
|
histogram |
|
|
inset table on plot |
|
|
statistical intervals |
|
|
output data set with summary statistics and capability indices |
|
|
probability-probability plot |
|
|
probability plot |
|
|
quantile-quantile plot |
You have three alternatives for producing plots with the CAPABILITY procedure:
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ODS Graphics output is produced if ODS Graphics is enabled, for example by specifying the ODS GRAPHICS ON statement prior to the PROC statement.
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Otherwise, traditional graphics are produced by default if SAS/GRAPH® is licensed.
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Legacy line printer charts are produced when you specify the LINEPRINTER option in the PROC statement.
See Chapter 3: SAS/QC Graphics, for more information about producing these different kinds of graphs.
You can use the INSET statement with any of the plot statements to enhance the plot with an inset table of summary statistics. The INSET statement is not applicable when you produce line printer plots.