Weighted averagesThe solution is to calculate a weighted average. A weighted average is an average where the results of some of the studies make a greater contribution to the total than others. All of the methods available for conducting metaanalyses in Cochrane reviews use forms of weighted averages. The various methods do this in different ways, and we will cover these methods in this module. In all methods, the underlying principle is to give more weight to studies that give us more information about the treatment effect. Sample size is the main factor in determining the weight for a trial. However, the event rate also makes a difference. This is because effects are generally estimated more precisely when there are lots of events. So, trials with higher event rates get more weight. At the extreme, a trial with no events tells us nothing about the effect of the intervention, and so gets no weight at all. The exact relationship between event rates and study weights is complex, and depends on the summary statistic being used. A statistical concept which takes into account both size of the study’s population, and its event rate, is variance. The box below provides an outline of the concept of variance. 

What is variance? 
We could assume that variance is inversely proportional to importance, i.e. the less variance in the study, the more weight it should contribute. One method, planned for RevMan 4.2 but not in earlier versions of RevMan, the inverse variance method, calculates study weights directly based on this assumption. There are other methods, called MantelHaenszel methods, which attribute weight in a manner closely related to inverse variance. In this module we will expand a little on the various available methods and look at some of the differences between them, finishing with some guidance on which method to use in your review. Within RevMan, the methods available are:


Read section 8.4 of the Reviewers’ Handbook  There is some information about the statistical techniques available in RevMan in Section 8.4 of the Reviewers’ Handbook and you should read it now.
In order to choose the method you are going to use in your metaanalysis, the first concept to understand is the difference between a fixed effect model and a random effects model. What does ‘fixed effect’ mean?To come up with any statistical model, or method for metaanalysis, we first need to make some assumptions. It is these assumptions that form the differences between all the methods listed above. A fixed effect model of metaanalysis is based on a mathematical assumption that every study is evaluating a common treatment effect. That means the effect of treatment, allowing for the play of chance, was the same in all studies. Another way of explaining this is to imagine that if all the studies were infinitely large they’d give identical results. The summary treatment effect estimate resulting from this method of metaanalysis is this one ‘true’ or ‘fixed’ treatment effect, and the confidence interval describes how uncertain we are about the estimate. Sometimes this underlying assumption of a fixed effect metaanalysis (i.e. that diverse studies can be estimating a single effect) is too simplistic. Therefore, the alternative approaches to metaanalysis are (i) to try to explain the variation or (ii) to use a random effects model. Random effects metaanalyses (DerSimonian and Laird)As we discussed above, fixed effect metaanalysis assumes that there is one identical true treatment effect common to every study.The random effects model of metaanalysis is an alternative approach to metaanalysis that does not assume that a common (‘fixed’) treatment effect exists. The random effects model assumes that the true treatment effects in the individual studies may be different from each other. That means there is no single number to estimate in the metaanalysis, but a distribution of numbers. The most common random effects model also assumes that these different true effects are normally distributed. The metaanalysis therefore estimates the mean and standard deviation of the different effects. By selecting ‘random effects’ in the analysis part of RevMan you can calculate an odds ratio, risk ratio or a risk difference based on this approach. The MantelHaenszel approachThe MantelHaenszel approach was developed by Mantel and Haenszel over 40 years ago to analyse odds ratios, and has been extended by others to analyse risk ratios and risk differences. It is unnecessary to understand all the details, but is sufficient to say that the MantelHaenszel method assumes a fixed effect and combines studies using a method similar to inverse variance approaches to determine the weight given to each study. The Peto methodThe Peto method works for odds ratios only. Focus is placed on the observed number of events in the experimental intervention. We call this O for ‘observed’ number of events, and compare this with E, the ‘expected’ number of events. Hence an alternative name for this method is the ‘O – E‘ method. The expected number is calculated using the overall event rate in both the experimental and control groups. Because of the way the Peto method calculates odds ratios, it is appropriate when trials have roughly equal number of participants in each group and treatment effects are small. Indeed, it was developed for use in megatrials in cancer and heart disease where small effects are likely, yet very important. The Peto method is better than the other approaches at estimating odds ratios when there are lots of trials with no events in one or both arms. It is the best method to use with rare outcomes of this type. The Peto method is generally less useful in Cochrane reviews, where trials are often small and some treatment effects may be large. 