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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 and
standard deviation and if each observation has approximately
the same weight (*w _{0}*),
then the least squares regression line has
approximately intercept and slope for vardef=WDF/WEIGHT
and slope 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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