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There are two summary statistics used for meta-analysis of continuous data, the mean difference (MD) and the standardized mean difference (SMD) (see Section 9.2.3). Selection of summary statistics for continuous data is principally determined by whether studies all report the outcome using the same scale (when the mean difference can be used) or using different scales (when the standardized mean difference has to be used).

The different roles played in the two approaches by the standard deviations of outcomes observed in the two groups should be understood.

· For the mean difference approach, the standard deviations are used together with the sample sizes to compute the weight given to each study. Studies with small standard deviations are given relatively higher weight whilst studies with larger standard deviations are given relatively smaller weights. This is appropriate if variation in standard deviations between studies reflects differences in the reliability of outcome measurements, but is probably not appropriate if the differences in standard deviation reflect real differences in the variability of outcomes in the study populations.

· For the standardized mean difference approach, the standard deviations are used to standardize the mean differences to a single scale (see Section 9.2.3.2), as well as in the computation of study weights. It is assumed that between-study variation in standard deviations reflects only differences in measurement scales and not differences in the reliability of outcome measures or variability among study populations.

These limitations of the methods should be borne in mind where unexpected variation of standard deviations across studies is observed.