QQ Ref Line

Distribution Analyses

QQ Ref Line

After choosing Curves:QQ Ref Line, you can use the QQ Ref Line dialog to add distribution reference lines to QQ plots.

Figure 38.35: QQ Ref Line Dialog

The default adds a least squares regression line. You can also specify your own reference line by choosing Method:Specification and specifying both the intercept and slope.

If you select a Weight variable, you can add a weighted least squares regression line to the normal QQ plot. If the data are normally distributed with mean $\mu$ and standard deviation $\sigma$ and if each observation has approximately the same weight (w₀), then the least squares regression line has approximately intercept $\mu$ and slope $\sigma$ for vardef=WDF/WEIGHT and slope ${\sigma/\sqrt{ w_0}}$ for vardef=DF/N.

A normal QQ plot with a least squares reference line is shown in Figure 38.36. Use the sliders to change the intercept and slope of the reference line.

Figure 38.36: Normal QQ Plot with a Reference Line

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