Meta-analysis is a powerful research tool, but some have been critical of the technique (e.g. Wilson & Rachman, 1983). Common criticisms include: the fact that reviews do not include single-case studies the inclusion of studies of questionable methodological adequacy the inclusion of studies not directly relevant to clinical issues, such as analogue studies, and trials of patients whose symptoms are not clinically significant or of great severity the fact that analyses can multiply sample measures taken from the same patient and from the same study leads to effect sizes computed on the basis of dependent data the fact that using average Z scores assumes that outcome measures are appropriately measured on an interval scale, and that their distribution may be assumed to have insignificant skewness and kurtosis sampling of studies will be biased by the tendency for editors and authors to favour positive results not all meta-analyses weight the means for sample size. A major difficulty is, however, that the effect size statistic can only speak to treatment effects for the average client, and though this is informative of general treatment effects, further elaboration of therapeutic impacts is usually required to detail the more specific effects of treatment. 7($X5&02!122$/*1.&'!+*.4!.4&!-2&!$,!2.1.*2.*/15!.&2.2!*#!%23/4$.4&(1%3! (&2&1(/4! 8<$*$72
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