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The CAPABILITY Procedure |
Figure 5.20.6 illustrates how a Q-Q plot is constructed. First, the nonmissing values of the variable are ordered from smallest to largest:
Then the th ordered value
is represented on the plot by a point whose
-coordinate is
and whose
-coordinate is
, where
is the theoretical distribution with zero location parameter and unit scale parameter.
You can modify the adjustment constants and
with the RANKADJ= and NADJ= options. This default combination is recommended by Blom (1958). For additional information, refer to Chambers et al. (1983). Since
is a quantile of the empirical cumulative distribution function (ecdf), a Q-Q plot compares quantiles of the ecdf with quantiles of a theoretical distribution. Probability plots (see PROBPLOT Statement) are constructed the same way, except that the
-axis is scaled nonlinearly in percentiles.
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