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Re: st: nl -choice between alternative parametrisations of sigmoid models
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From |
Steven Joel Hirsch Samuels <[email protected]> |
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To |
[email protected] |
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Subject |
Re: st: nl -choice between alternative parametrisations of sigmoid models |
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Date |
Thu, 25 Oct 2007 12:05:32 -0400 |
Rosy, are you saying that the same set of data will produce two
different curves? What conditions vary to do this? Exactly what are
the data? Have you plotted the data? Are you sure that the curve is
monotone?
-Steven
On Oct 25, 2007, at 11:41 AM, Rosy Reynolds wrote:
Hello,
I am fitting 4-parameter logistic (sigmoid Emax) dose-response
models using the built-in -log4- feature of -nl-.
The model is y= b0 + b1/(1 + exp(-b2*(x-b3))) + error
and the coefficients can be interpreted as
b0 = baseline outcome
b1 = Emax i.e. largest change from baseline
b2 = Hill or slope coefficient
b3 = ED50 i.e. value of x (dose) required to produce half-maximal
effect,
The same curve can actually be produced with two different sets of
these parameters.
In one set, the Hill slope b2 is positive and the other parameters
intuitively have the interpretations above.
In the other set, b2 is negative, the sign of b1 is reversed, and
b0 becomes the outcome at infinitely high dose instead of at the
lowest doses. The lowest-dose outcome is now given by b0+b1.
With our data, -nl- naturally produces the set of coefficients with
negative b2.
For ease of interpretation, I would prefer the set with positive b2.
I can push -nl- into doing that by supplying carefully chosen
starting values close to the desired coefficients. I could even run
-nl- , manipulate the coefficients it obtains, and use those as
starting values. That would always work, I suppose, but it seems
long-winded.
Please could you tell me an easier way to make -nl- parametrise the
model in the preferred way, if you know of one? I haven't found
anything about it in the manual.
Thanks for thinking about this.
best wishes
Rosy Reynolds
BSAC Resistance Surveillance Co-ordinator
www.bsacsurv.org
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Steven Samuels
[email protected]
18 Cantine's Island
Saugerties, NY 12477
Phone: 845-246-0774
EFax: 208-498-7441
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* http://www.ats.ucla.edu/stat/stata/