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Year of publication of studies (n = 145)
| Year | Number of studies |
|---|---|
| 1974 | 1 |
| 1975 | 1 |
| 1976 | 0 |
| 1977 | 0 |
| 1978 | 0 |
| 1979 | 3 |
| 1980 | 4 |
| 1981 | 4 |
| 1982 | 3 |
| 1983 | 2 |
| 1984 | 4 |
| 1985 | 3 |
| 1986 | 2 |
| 1987 | 7 |
| 1988 | 4 |
| 1989 | 3 |
| 1990 | 8 |
| 1991 | 5 |
| 1992 | 3 |
| 1993 | 2 |
| 1994 | 10 |
| 1995 | 15 |
| 1996 | 13 |
| 1997 | 12 |
| 1998 | 8 |
| 1999 | 13 |
| 2000 | 15 |
