New answers tagged nonlinear-regression
I assume that the log(S) vs. Y curve is symmetrical, so the uncertainty of the log(IC50) is symmetrical and the uncertainty of the IC50 is not. Accordingly, you are much better off using your second model. You'll get the same the best-fit curve, the same best-fit value for the IC50 (after transforming), and the same sum-of-squares and R2. But the two ...
Are you just passing the lower triangle into the correlation structure? Your error is saying your correlation matrix is not positive-definite. For a matrix to be positive definite, it will be nxn and symmetric. Pass the whole matrix to the function... Otherwise, you're likely going to have to post the matrix for more help.
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