# Re: st: meta-regression coefficient - adjusting?

 From Roger Harbord <[email protected]> To [email protected] Subject Re: st: meta-regression coefficient - adjusting? Date Sun, 05 Dec 2004 16:37:08 -0000

```--On 01 December 2004 14:31 -0500 ronald eysenck <[email protected]> wrote:

```
```I am performing a meta-regression analysis using 'metareg.' When no
covariates are in the model, the intercept should be the exact equivalent
of a random-effect pooled estimate calculated with 'meta.'

Now, the purpose of these models are: [1] to look at predictors of the
effect estimate (or in other words, to ceck if any study-feature makes
studies more likely to show a bigger or smaller effect size); [2] ...here
is my doubt: how should we interpret the intercept in a metaregression
model with covariates? Should it be considered as a pooled estimate
say, in a logistic regression model? I have been taking a look in books and
articles on the subject but have not been able so far to come up with a
```
--On 03 December 2004 10:14 +0100 "G. ter Riet" <[email protected]> replied:

```My understanding is that the intercept in a model with covariates
represents the pooled estimate in the subgroup in which all covariates
have the value of zero. I do not know a reference on that.
```
I'd agree with that. Metaregression examines how the effect varies according to the covariate, it doesn't adjust for the covariate. This is no different from ordinary regression - by regressing y on x you're examining how y varies with x, not adjusting y for x. If y does vary with x it is not very useful to give a single value summarising y for all values of x. The intercept is simply the fitted value of y when x=0 and so isn't of any greater interest than the fitted value of y at any other value of x. That's why the intercept is seldom the focus of interpretation.

A good reference on how meta-regression analysis should be conducted and interpreted is:

SG Thompson and JPT Higgins, "How should meta-regression analyses be undertaken and interpreted?",Stat Med 2002;21(11):1559-1573

Roger.

--
Roger Harbord
Department of Social Medicine, University of Bristol, UK
http://www.epi.bris.ac.uk

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