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# Re: st: Modelling Relative Risks with -fracpoly-

 From Nick Cox To statalist@hsphsun2.harvard.edu Subject Re: st: Modelling Relative Risks with -fracpoly- Date Wed, 20 Mar 2013 15:12:51 +0000

```Sorry, that was too hasty. You said much more than I noticed in a

However, forcing through the origin here still seems more problematic
than usual.

In the easiest applications, (0, 0) is unattainable but a sensible
limit on physical (biological, economic, ...) grounds. (Mundane
example: length and area of objects.) In the best applications,
forcing a function through the origin is also consistent with the data
say.

Here it seems that RR < 1 and RR > 1 could be something you observe
even for exposure at or near 0, just as a matter of empirical
reflect that any way. If they aren't equally common, force is not
nice.

Nick

On Wed, Mar 20, 2013 at 3:03 PM, Nick Cox <njcoxstata@gmail.com> wrote:
> 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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