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## 4PL fit (4 parameter logistic)

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).

1 ACCEPTED SOLUTION

Accepted Solutions
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## Re: 4PL fit (4 parameter logistic)

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.

-Michael

Michael Crotty
Sr Statistical Writer
JMP Development
6 REPLIES 6
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## Re: 4PL fit (4 parameter logistic)

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

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

Highlighted  Jeff_Perkinson Community Manager

## Re: 4PL fit (4 parameter logistic)

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

-Jeff

-Jeff
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## Re: 4PL fit (4 parameter logistic)

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

Highlighted

## Re: 4PL fit (4 parameter logistic)

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.

-Michael

Michael Crotty
Sr Statistical Writer
JMP Development
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## Re: 4PL fit (4 parameter logistic)

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

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## Re: 4PL fit (4 parameter logistic)

Thanks a lot. Those are great answers.