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Hi Dominic, To me, it all depends on the protocol of the review. Deviations from the original protocol after searching for studies, usually introduce bias in the SR. You should restrict yourself to your original PICO and to what you wrote in the protocol. If it was intervention A versus B in population X. So, don't combine results from mixed populations (X + Y) especially if the original authors gave results for a mixed population without sub-grouping it in the original study (i.e. randomized each population separately and calculated its results separately). Obviously this would disrupt the original randomization and introduce bias. Another important point is that clinical homogeneity of the combined studies is an essential prerequisite for a good metaanalysis. Best of wishes, Abdelhamid Attia Prof. Of Obstetrics & Gynecology; Cairo University Assistant Secretary General Of The Egyptian Fellowship President Of The Arab Federation of Evidence-Based Medicine -----Original Message----- From: Evidence based health (EBH) [mailto:[log in to unmask]] On Behalf Of Dominic Hurst Sent: Thursday, 13 January, 2011 5:32 PM To: [log in to unmask] Subject: Combining results for meta-analysis Hi, I'd appreciate some help on the following: A systematic review looks to compare the effectiveness of two interventions, A and B, in a particular population, X. The interventions, though, are commonly used in a discrete population Y also. Some of the studies retrieved compare A and B just in the desired population X, but others compare the interventions in a mix of populations X and Y. In the latter there may not have been block randomisation so the proportions of X and Y receiving A or B may be unbalanced. In doing a meta-analysis of these studies, should one be cautious in looking to combine the results from the X-only studies with those extracted from the X-Y mixed studies? Does it matter that in removing the subgroup X from the mixed study the original randomisation has been disrupted and does it matter that the A and B intervention groups may be then be unbalanced? Would it be reasonable to test for the significance of this with sensitivity analysis by removing the results from the mixed studies after the meta-analysis? Thanks, Dominic __________ Information from ESET Smart Security, version of virus signature database 5782 (20110112) __________ The message was checked by ESET Smart Security. http://www.eset.com __________ Information from ESET Smart Security, version of virus signature database 5782 (20110112) __________ The message was checked by ESET Smart Security. http://www.eset.com