Ratios as DVs and IVs: BMI (was Re: Interesting results, how to deal with these interaction effects?)

[Peter Flom]

Thanks to Frank Harrell and Bruce Weaver for their insights and
recommendations.

In the article by Kronmal to which Bruce refers below, he argues against any use
of a ratio DV or IV

This got me thinking about studies that use BMI, either as DV or IV. BMI is, of course,
a ratio.

Are these studies therefore bogus?

Peter

-----Original Message-----
>From: Bruce Weaver <bwe...@lakeheadu.ca>
>Sent: Oct 23, 2009 10:16 AM
>To: MedStats <meds...@googlegroups.com>
>Subject: {MEDSTATS} Re: Interesting results, how to deal with these interaction effects?
>
>
>On Oct 23, 9:26 am, Frank Harrell <f.harr...@vanderbilt.edu> wrote:
>> The laws of arithmetic are violated any time you use % change as a
>> dependent variable.  A simple way to see this is that an increase of
>> 100% is balanced by a decrease of 50%.  Percent changes may only be
>> computed on group summary measures, not per-patient measures.
>
>Peter, here are a couple articles that might help you argue Frank's
>point about % change with the investigators.
>
> http://www.jstor.org/stable/2983064
> http://www.biomedcentral.com/1471-2288/1/6
>
>If you don't have JSTOR access, I can send you the Kronmal article--
>just drop me a line.
>
>--
>Bruce Weaver
>bwe...@lakeheadu.ca
>http://sites.google.com/a/lakeheadu.ca/bweaver/Home
>"When all else fails, RTFM."
>
>>


Peter L. Flom, PhD
Statistical Consultant
Website: www DOT peterflomconsulting DOT com
Writing; http://www.associatedcontent.com/user/582880/peter_flom.html
Twitter: @peterflom