Results
A total of 812 unduplicated papers on relevant subjects were identified from the search strategy of which 791 were excluded because they did not meet inclusion criteria. The number of studies excluded at each stage and reasons exclusion of studies in which full texts were obtained are shown in figure 1. The final pool consisted of 21 observational studies reporting a wide range of study designs and interventions. No studies were rated as ‘strong’, six were classified as ‘moderate’ and 15 ‘weak’. Due to the limited evidence base, no studies were excluded from the results. General characteristics of included studies are summarized in table 1. Details of all the studies including intervention type and quality rating can be found in the Supplementary
Appendix.