Jay P Sah wrote: > Recent discussion on how to compare regression lines was really = > enlightening. As far as I understood, the discussion was about comparing = > slopes in linear regressions.=20 > > =20 > > I have similar problems in comparing non-linear regression lines. Here, = > it goes what I want to compare. I have shrub biomass and pine seedling = > density data collected in 160 50-m^2 plots. I used non-linear regression = > (y =3D b0*(1-exp(-b1^x)) to model the cumulative seedling density = > against shrub biomass. I am interested to test if this regression curve = > (observed pattern) differs from another similar curve (a reference = > model), generated by using Poisson cumulative distribution function. My = > questions are: how to compare these two non-linear regression lines or = > the coefficients there in? Are the methods described in Zar's book and = > elsewhere, more commonly used for comparing slopes in linear regression, = > also applicable for non-linear regression lines? =20 > > If the two models are nested, then you can compare them using an F test (I'm not sure if the results are exact, but they should be OK with a decent amount of data). If they're not nested, but have the same number of parameters, then you can compare the residual sums of squares: the smallest wins! If they have different numbers of parameters and are not nested, then there are several ways of comparing them, for example, using AIC, BIC etc.
As a starter, I would recommend plotting the residuals against the predicted values: it may be that it's obvious that one curve does not fit. Or indeed that neither curve fits! Incidentally, I'm not sure what you mean by using the Poisson cdf: that's a stepped function, but it sounds like your data are continuous. Bob -- Bob O'Hara Department of Mathematics and Statistics P.O. Box 68 (Gustaf Hällströmin katu 2b) FIN-00014 University of Helsinki Finland Telephone: +358-9-191 51479 Mobile: +358 50 599 0540 Fax: +358-9-191 51400 WWW: http://www.RNI.Helsinki.FI/~boh/ Journal of Negative Results - EEB: www.jnr-eeb.org
