4PL fit (4 parameter logistic)

[#1]

This may be a naïve question but I am confused about the 4PL equation used by jmp in curve fitting. I have a dataset of two variable and I choose: analyze>modeling>nonlinear

Then I plot the data and the red fit-curve button gives the option: Sigmoid curves>logistic curves>Fit logistic 4P

Now my question is; the equation used by jmp is f(x).

[#2]

It seems when I posted half my post was deleted??

...now my question is; the equation used by jmp is f(x)

[#3]

Sorry you had trouble with your post. It seems though that there's still something missing.

Can you clarify your question?

-Jeff

[#4]

Yes it seems everything after the equals sign gets deleted. Thanks for replying. My question concerns the logistic 4P fit in sasjmp. The equation used for this is not the same as for 4 parameter logistic fits used in various applications software I am used to. Is it just not the same model or is there some mathematical point I am missing? Usually 4PL is described as y equals ((A-D)/(1+((x/C)))+D

[#5 · Accepted solution]

Yes, the parameterization you give is different than the one used in JMP. The following is a reasonable discussion of the two parameterizations. JMP uses the one that it does to avoid having a parameter in an exponent, which makes estimation more difficult.

http://stats.stackexchange.com/questions/9298/reconciling-different-parameterizations-of-the-same-no...​

-Michael

[#6]

Hello,

I looked through the Nonlinear Model Library and I found the the 4P logistic model you describe is called the Rodbard Model (4P) in JMP.

Best,

Bill

[#7]

Thanks a lot. Those are great answers.