[R] Restricted 4-PL curves using drc package

[Bert Gunter]

1. Don't know, but, assuming I understand you correctly, probably not.

2 Note (subject to my understanding again) that you can rephrase your
query as fitting a 4 parameter logistic to all the data together with
a grouping factor to indicate the separate curves, allowing two of the
parameters (e.g. ED50 and slope) to vary by group. You can then use
standard nonlinear fitting (e.g. via the optimx package) to fit the
model. Note that you will have to specify 2c +2 starting values for c
curves. With c "large" (?whatever that means), this may make
convergence tricky.

If you care to reply, please do so to the list, not to me.

Cheers,
Bert




Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Wed, Dec 9, 2015 at 12:27 PM, li li <hannah.hlx at gmail.com> wrote:
> Hi all,
>   In drc package, is there a function which can be used to fit restricted
> 4PL curves? For example, we restrict two 4PL curves have the same lower and
> upper asymptotes?
>   Thanks for the help in advance!
>     Hanna
>
>         [[alternative HTML version deleted]]
>
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