Originally Posted by

**ohammer**
Correct, if you want to fit the logistic function using linearization, you must estimate *a* independently. Honestly I don't quite understand what transformation SPSS did for you (it says the dependent variable is (1/SEAsia), which I can't quite fit in with the logistic model?).

If you use a nonlinear procedure instead, you can fit all three parameters simultaneously.

(The program Past does this automatically by first setting a to the max value of the data as an initial guess, then estimating b and c by linearization and regression, then optimizing all the parameters with the Levenberg method).