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Table 1 Overview of included studies.
| First author year | from | N | Participants | Sampling | Response rate | Focus pure/impure placebo |
|---|---|---|---|---|---|---|
| Questionnaire-based quantitative surveys | ||||||
| Shapiro 1973 [15, 16] | USA | 195 | Ph (various groups) | Convenience | 83% | Both |
| Goldberg 1979 [18] | USA | 102 | N (head nurses at 11 hospitals) | Local | 68% | Pure |
| Goodwin 1979 [19] | USA | 60 | Ph (house officers) | Convenience | 100% | Mainly |
| 39 | N (hospital nurses) | Convenience | Unclear | pure | ||
| 27 | License practical nurses/medical aides | Convenience | Unclear | |||
| Gray 1981 [20] | CAN | 70 | Ph (university hospital) | Convenience | 82%* | Pure |
| 230 | N (university hospital) | Convenience | ||||
| 35 | N (experienced hospital nurses) | Convenience | Unclear | |||
| Lange 1981 [21] | GER | 81 | Ph, N, psychologists (no data for subgroups provided) | Convenience | Unclear | Pure |
| Thomson 1982 [22] | NZ | 37 | Ph (GPs) | Local | 84% | Both |
| Classen 1985 [23] | GER | 101 | Ph (setting unclear) | Convenience | Unclear | Both |
| Saupe 1986 [24] | GER | 56 | N (at a psychiatric university hospital) | Convenience | 80% | Mainly pure |
| Lynöe 1993 [26] | SWE | 94 | Ph (GPs or affiliated with university) | Local | 94% | Mainly |
| 83 | Pt (consecutive patients of three GPs) | Local | 83% | impure | ||
| Ernst 1997 [27] | AUS | 263 | N (setting unclear) | Convenience | 58% | Both |
| Berger 1999 [28] | USA | 74 | Medical interns at an university-affiliated hospital | Convenience | 83% | Pure |
| Berthelot 2001 [29] | FRA | 300 | Pt (at a hospital rheumatology department) | Convenience | Unclear | Mainly |
| 100 | N (same hospital, various departments) | Convenience | Unclear | pure | ||
| Hrobjartsson 2003 [30] | DEN | 502 | Ph (GPs, hospital, specialists in private practice) | Random | 65% | Both |
| Nitzan 2005 [31] | ISR | 31 | Ph (senior hospital physicians) | Convenience | 76% | Mainly |
| 31 | N (head nurses from same hospitals) | Convenience | 100% | pure | ||
| 27 | Ph (family physicians) | Convenience | 68% | |||
| Lim 2007 [32] | SIN | 402 | Medical students | Local | 36% | Pure |
| Sherman 2007 [33] | USA | 231 | Ph (faculty members of 3 medical schools) | Local | 50% | Both |
| Tilburt 2008 [34] | USA | 679 | Ph (internists and rheumatologists) | Random | 57% | Both |
| Bernateck 2009 [35] | GER | 71 | Ph (university hospital) | Convenience | 80%* | Pure |
| 107 | N (university hospital) | Convenience | ||||
| Chen 2009 [36] | NZ | 211 | Pt (in waiting rooms of two GP clinics) | Convenience | 48% | Mainly pure |
| Fässler 2009 [37] | SWI | 233 | Ph (primary care) | Random | 48% | Both |
| Substudies with prospective screening of medical records of hospital patients | ||||||
| Goodwin 1979 [19] | USA | 1900 | Pt (academic teaching hospital, treated during 6 months) | n.a. | Pure | |
| Lange 1981 [21] | GER | 1725 | Pt (psychiatric hospital, all treated 1978) | n.a. | Pure | |
| Qualitative studies | ||||||
| Comaroff 1976 [17] | UK | 47 | Ph (GPs) | Local | 92% | Mainly impure |
| Schwartz 1989 [25] | USA | 72 | Ph (selected for often prescribing inefficient drugs) | n.a. | 51% | Impure |
