SF updated the review (July 2006).

RG and SF designed the protocol and wrote the research proposal for application for funding. RG commented on the protocol and review.

SF was involved in the designing and running of the search strategy, screened titles and abstracts for eligibility, extracted data, critically evaluated and quality assessed the included studies and wrote up the review. SF supervised the work of research assistant, DLO.

DLO assisted in the writing of the protocol and in the designing of the search strategy. Advice on the search strategy was provided by Martin Hewitt, information specialist, King's College. DLO screened titles and abstracts for eligibility, obtained references, contacted authors for further information, extracted data, quality assessed the included studies and assisted in the writing of the review.