#### 9.4.4.1 Mantel-Haenszel methods

The Mantel-Haenszel methods (Mantel 1959, Greenland 1985) are the default fixed-effect methods of meta-analysis programmed in RevMan. When data are sparse, either in terms of event rates being low or study size being small, the estimates of the standard errors of the effect estimates that are used in the inverse variance methods may be poor. Mantel-Haenszel methods use a different weighting scheme that depends upon which effect measure (e.g. risk ratio, odds ratio, risk difference) is being used. They have been shown to have better statistical properties when there are few events. As this is a common situation in Cochrane reviews, the Mantel-Haenszel method is generally preferable to the inverse variance method. In other situations the two methods give similar estimates.