Bug: Incorrect results from test for homoscedasticity as part of nonlinear regression

Last modified October 11, 2017

Prism 7 added the ability to test the residuals from nonlinear regression for homoscedasticity (equal variance). This is also called a test for appropriate weighting. This test computes the nonparametric correlation between the absolute value of the weighted residuals and the predicted Y values (the Y values of the fit curve). If you picked an appropriate weighting, there should be low correlation and a large P value. 

Prism computes this test differently when the sample size (n) is 16 or less, and when n is 17 or more. There are bugs in both.  These bugs are in Prism 7 Windows up to 7.03 an Prism Mac up to 7.0c. They are fixed in 7.04 and 7.0d. 

If n<= 16

  • The reported P value is correct.
  • Regardless of P value, it always concludes that the data passed the test (reports "yes")

If n > 16

  • The P value is always reported as P>0.9999
  • The conclusion about statistical significance, whether the data passed the test, is correct

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