Calculating R^2 for a nonlinear least squares fit

Question

Suppose I have x values, y values, and expected y values f (from some nonlinear best fit curve).

How can I compute R^2 in R? N

Answer 1

As an alternative to this problem I used at several times the following procedure:

1. compute a fit on data with the nls function
2. using the resulting model make predictions
3. Trace (plot...) the data against the values predicted by the model (if the model is good, points should be near the bissectrix).
4. Compute the R2 of the linear régression.

Best wishes to all. Patrick.