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Re: st: Modelling Relative Risks with -fracpoly-


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Modelling Relative Risks with -fracpoly-
Date   Wed, 20 Mar 2013 15:03:48 +0000

What is the origin here?

Normally something we should all have been able to answer at age 13 or
so, but please bear with me.

If logRR = 0 then RR = 1.

If RR = 0 then logRR is indeterminate.

Do you want either limiting behaviour?

If so, why? If not, what else?

Either way, you could try choosing a set of powers that had the
behaviour you want, but that might get in the way of -fracpoly-'s
scope for adjusting to the data.

Nick

On Wed, Mar 20, 2013 at 2:52 PM, Colin Angus <c.r.angus@sheffield.ac.uk> wrote:

> I'm using the -fracpoly- command to model the log relative risk of an
> event as a function of a single continuous exposure variable, where
> the reference category for my relative risk is those with an exposure
> of 0 (i.e. my log RR at 0 exposure is 0). So my command is:
>
> -fracpoly: regress logRR exposure [weight=weight]-
>
> I cannot see how to force the fitted fractional polynomial function
> through the origin. Even if I use the -nocon- command to supress the
> constant term, the transformations of the exposure variable mean that
> the fitted value at 0 isn't 0.
>
> Can anybody help me?
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