If the data are exact and pooled (using a system variable), and if the shape is estimated from the data, then Bartlett's modified likelihood ratio test provides a test for equality of the shape parameters. The test statistic is calculated as follows:
where
Test across growth curves
If the data are exact and grouped (using a By variable), and if the shape is estimated from the data, then a likelihood ratio test provides a test for equality of the shape parameters. The test statistic is calculated as follows:
when 
otherwise 