[R] testing parallelism of does-response curves using nls()

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Hi, I am trying to use F test or Chi-square test to test if 2 5-parameter (A, B, xmid, scal and H) logistic curves are parallel based on residual sum of squares.

What's usually done is to first fit the 2 curves using a constraint (or global) model where all parameters are kept the same except for "xmid"; then fit 2 independent curves using unconstraint models where all 5 parameters are allowed for change. The extra residual sum of squares can then be used to test parallelism using either Chi-square or F test.

Now, fitting 2 separate curves are straight forward using nls(), but how to fit a global model to only allow "xmid" to be different among the 2 curves?

BTW, is there a selfStart function for 5-parameter logistic like SSfpl for 4-parameter logistic function?

Thanks very much,

John