array chip

2010-Mar-13 01:05 UTC

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

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

Eik Vettorazzi

2010-Mar-13 14:12 UTC

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

Hi John, you may have a look at the drc-package (or look at the maintainers web site www.bioassay.dk) I think it does exactly what you want. HTH Eik array chip schrieb:> 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 > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >