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st: predictnl and nonlinear regression


From   "Wallace, John" <John_Wallace@affymetrix.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: predictnl and nonlinear regression
Date   Fri, 5 Aug 2005 17:02:45 -0700

Dear listmembers:
I'm working my way through learning how to use regressions other than OLS within stata.  One model I frequently use in Excel is the exponential fit.  I believe the following is the way to specify that model within Stata's nl function:
nl(y = {beta}*exp({alpha}*x)
I have used the predict function following this estimation to generate a fit and graphed the results, which look like the chart I was expecting to see.  I then looked for a way to generate confidence intervals for the fit, and have run aground.  It looks like the predictnl function would do what I want, but I can't get it to work.  Here's what I've tried:
(after first -drop-ing the fit variable I made with -predict-)

. predictnl fit = predict(fit), ci(ub lb)
	Result: fit not allowed \\\ predict(fit) invalid
. predictnl fit = nl (y = {beta}*exp({alpha}*x)), ci(ub lb)
	Result: Unknown function nl()
. predictnl fit = y = {beta}*exp({alpha}*x), ci(ub lb)
	Result: =exp not allowed

Also, is it possible to use the stored estimation command interactively to solve for a specific x?
e.g.: . predicti(0.5)
which returns some value y according to the model?


John Wallace | Research Associate | Test Method Development
AFFYMETRIX, INC. | 3380 Central Expressway | Santa Clara, CA 95051 | Tel:  408-731-5574 | Fax:  408-481-0435


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