130 Chapter 5 Six were prospective cohort studies, one combined a prospective- with a retrospective cohort, four were multicentre retrospective cohort studies, thirty-eight were retrospective cohort studies and one was a matched case-control study. None of the studies had a randomized design. Nineteen of the studies were conducted in North America, sixteen in Europe, eleven in Asia, three in Africa and one in the Middle-East. Follow up was at least half a year in 24 (48%) of all studies. Figure 1 - Flow diagram article selection
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