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RE: st: Discrepancy between metan vs metareg with one variable


From   "Trelle Sven" <strelle@ctu.unibe.ch>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Discrepancy between metan vs metareg with one variable
Date   Wed, 19 Dec 2012 11:55:28 +0100

If my understanding is correct I would assume that this is because a
meta-regression estimates one random-effect (between-trial
heterogeneity; one tau-square==variance of the underlying normal
distribution) whereas a sub-group analysis using metan estimates
separate random-effects for each subgroup (n tau-squares).
Hence,interpretation is different for these two approaches. You will see
large differences between the two approaches if the stratification is
able to explain heterogeneity only in some subgroups but not all.

To replicate the test-statistic you would need to force metan to
estimate only one tau-square (across the subgroups) but I wouldn't know
how to do this.

Best
Sven TRelle


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Paul Karner
Sent: Dienstag, 18. Dezember 2012 18:09
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Discrepancy between metan vs metareg with one variable

Thanks for your reply.

They can be quite different -- in one instance by more than 10%.  I've
played around with the different options for specifying the method of
variance estimation and that does not resolve the large discrepancy.

Potentially related, I notice when calculating the test statistic for
the difference between effect sizes for two subgroups that the z-stat
from the meta regression is much smaller than the hand-calculated z-stat
I get [using the pooled standard error estimate
sqrt(se_group1^2 + se_group0^2)].  This is only true for random-effects
meta regression (i.e. the hand-calculated z-stat is identical to that
reported by "vwls").  Is this because the pooled standard error on which
coefficient test statistics are based in random effects meta regression
somehow also account for between-study heterogeneity?  If so, does
anyone know exactly how one would replicate the z-stats reported by
metareg, z by hand?

Thank you again for your insight!

Paul

On Tue, Dec 18, 2012 at 10:09 AM, JVerkuilen (Gmail)
<jvverkuilen@gmail.com> wrote:
> On Tue, Dec 18, 2012 at 9:30 AM, Paul Karner <pkarner@gmail.com>
wrote:
>> I have a question about Stata's (user-written) meta analysis
functions.
>>
>> When I run metareg with a single variable ("group1"), why is the 
>> coefficient estimate on group1 slightly different from what I get 
>> when I run metan, random, by(group1) and subtract the effect 
>> estimates for the two subgroups (i.e., group1==1 and group1==0)?
>
> How different is different? The two methods may be using slightly 
> different estimators as defaults, so for instance if one is using Der 
> Simonian-Laird and the other REML, you're going to have differences.
> You need to check to see if you're estimating the model the same way 
> and then compare. Even then they may well not line up perfectly if 
> there are other slight differences in the programming between the two 
> procedures, but they should be on par up to about four decimal places.
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