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st: re. study weight in meta analysis


From   "roger webb" <roger.webb@man.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   st: re. study weight in meta analysis
Date   Fri, 12 Mar 2004 11:40:11 -0000

Dear Statalist

Iím conducting a meta-analysis for population-based studies of a 
rare mortality outcome (stillbirths) using the Ďmetaní procedure.

My question concerns the derivation and interpretation of the study 
weights in the final column of this Stata output:

. metan ideath inodeath cdeath cnodeath, rr 
label(namevar=studyno)

        Study |       RR  [95% Conf. Interval]  % Weight
--------------+-----------------------------------------
1             |      3.5   1.04399   11.7339   6.34589
2             |        4     .4517   35.4217   2.37971
3             |  1.62944   1.02163   2.59886   51.6439
4             |  2.50294   1.25479   4.99266   15.2031
5             |  3.95477   1.15619   13.5274   4.80286
6             |  .810097   .282934   2.31947   19.6245
--------------------------------------------------------
MH pooled RR  |  1.88825   1.36146   2.61886
--------------------------------------------------------
  Heterogeneity chi-squared =   6.35 (d.f. = 5) p=0.273
  Test of RR=1 : z= 3.81 p = 0.000

The raw data for studies 4 and 6 are as follows:

Study 4:
(RR=2.50; study weight: 15.2%)
Exposed cohort: deaths (n=8); survivors (n=927)
Unexposed cohort: deaths (n=5,319); survivors (n=1,550,656)

Study 6:
(RR=0.81; study weight: 19.6%)
Exposed cohort: deaths (n=4); survivors (n=614)
Unexposed cohort: deaths (n=25); survivors (n=3,104)

The sample sizes and numbers of events are smaller in Study 6 
than in Study 4 and yet Study 6 has the larger study weighting. 
Could anyone kindly provide an explanation for this? Thanks.

Regards
Roger Webb
University of Manchester, UK



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