Methods and formulas for test for equal shapes or scales in Parametric Growth Curve

Bartlett's modified likelihood ratio test

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:

where

when

otherwise

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