Attrition does not generally present a significant threat of bias as follow-up rates were in excess of 80% in all studies. Perhaps of greater relevance were the various forms of selection employed in sample construction, particularly for the smaller studies, which mean that generalisability of findings must be carefully considered. The definition of the exposure, AFD, among included studies is robust, with studies excluded if this was problematic.23 24 Similarly outcome measurement is reliable, generally making use of validated measures, and where there are minor deviations from Diagnostic and Statistical Manual of Mental Disorders, for example in timeframes used, these are well justified.
