Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: st: Gompertz equation
From
Nick Cox <[email protected]>
To
[email protected]
Subject
Re: st: Gompertz equation
Date
Wed, 23 Mar 2011 10:39:17 +0000
Fitting models with -nl- can be tricky even when the model and the
data are well matched. I've found that giving reasonable initial
values can make all the difference between an excellent fit and no fit
at all. In your case I'd worry about the numerics of exp(exp()) and
scale so that x and y were say of order 1 rather than something much
bigger or smaller.
I note that your syntax as stated wouldn't work without {} around parameters.
Nick
On Wed, Mar 23, 2011 at 12:24 AM, Michael Begg <[email protected]> wrote:
> I am trying to estimate the equation y = b1*exp(b2*exp(b3*x)) using the
> following:
>
> nl (y = b1*exp(b2*exp(b3*x)))
>
> Stata takes b3 as a constant and b2 is omitted.
>
> Why is this happening?
>
> If I use the inbuilt gompertz function form y = b1*exp(-exp*(b2*(b3 - x)))
> using
>
> nl gom3: y x
>
> it works fine.
*
* For searches and help try:
* http://www.stata.com/help.cgi?search
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/