The overall intervention effect can also be difficult to interpret as it is reported in units of standard deviation rather than in units of any of the measurement scales used in the review, but in some circumstances it is possible to transform the effect back to the units used in a specific study (see Chapter 12, Section 12.6). For example, where pragmatic and explanatory trials are combined in the same review, pragmatic trials may include a wider range of participants and may consequently have higher standard deviations. This assumption may be problematic in some circumstances where we expect real differences in variability between the participants in different studies. ![]() However, the method assumes that the differences in standard deviations among studies reflect differences in measurement scales and not real differences in variability among study populations. ![]() Thus studies for which the difference in means is the same proportion of the standard deviation will have the same SMD, regardless of the actual scales used to make the measurements. (Again in reality the intervention effect is a difference in means and not a mean of differences.): The standardized mean difference expresses the size of the intervention effect in each study relative to the variability observed in that study. In this circumstance it is necessary to standardize the results of the studies to a uniform scale before they can be combined. The standardized mean difference is used as a summary statistic in meta-analysis when the studies all assess the same outcome but measure it in a variety of ways (for example, all studies measure depression but they use different psychometric scales). For the current version, please go to /handbook/current or search for this chapter here. This is an archived version of the Handbook.
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