Underlying assumptions of metaanalysis

The underlying assumption of meta-analysis is that in a group of randomized trials addressing similar questions, the true differences between treatments are likely to be in same direction. The observed size of effect may vary from trial to trial and in a small number of cases the direction may be reversed by chance alone. Although we cannot assume that the effect of a treatment given in different trials to different patient populations will be to produce exactly the same risk reduction, it is reasonable to assume that the net effect of treatment will tend to be in the same direction. Although the direction of effect maybe obscured in any one trial, when all the evidence is considered, the benefit or detriment of treatment will be reflected in the overall pattern of individual trial results. The combination of these results provides an estimate of the 'average' treatment effect.

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