RCT | Nayir21 | | 2001 | Turkey | Hospital outpatients | | 70 | 3 | | 3 months to 10 years |
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Cohort studies | Schoen et al22 | | 2000 | USA | Hospital in/outpatient | | 14 893 | 154 | | <1 year |
| To et al1 | | 1998 | Canada | Hospital in/outpatient* | | 58 434 | 330 | | <3 years |
| Wiswell and Hachey6 | | 1993 | USA | Hospital inpatient | | 107 598 | 496 | | <1 year |
| Wiswell et al5 | | 1987 | USA | Hospital inpatient | | 219 775 | 610 | | <1 year |
| | | | | | | | | | |
Case–control studies | Craig et al23 | | 1996 | Australia | Hospital in/outpatient | | 886 | 144 | | <5 years |
Newman et al24 | | 2002 | USA | Non-hospital outpatients | | 769 | 56 | | <3 months |
| Rushton and Majd25 | | 1992 | USA | Hospital inpatient | | 86 | 23 | | <6 months |
| Spach et al26 | | 1992 | USA | Community sexually transmitted diseases clinic | | 78 | 26 | | Adult |
| Crain and Gershel27 | | 1990 | USA | Hospital outpatient | | 81 | 22 | | <2 months |
| Kashani and Faraday28 | | 1989 | USA | Hospital inpatient | | 126 | 17 | | 1 month to 2 years |
| Herzog29 | | 1989 | USA | Hospital outpatient | | 112 | 36 | | <1 year |