| Year | Value | | 2004 | 27.386091127098 | | 2003 | 36.165577342048 | | 2002 | 36.405886909373 | | 2001 | 41.371841155235 | | 2000 | 43.789308176101 | | 1999 | 47.195512820513 | | 1998 | 48.139711465452 | | 1997 | 45.603944124897 | | 1996 | 41.054451166811 | | 1995 | 45.118483412322 | | 1994 | 46.999076638966 | | 1993 | 47.801147227533 | | 1992 | 50.571131879543 | | 1991 | 46.285714285714 | | 1990 | 44.121212121212 | | 1989 | 48.803191489362 | | 1988 | 49.779086892489 | | 1987 | 45.280235988201 | | 1986 | 50.406504065041 | | 1985 | 51.668726823239 | | 1984 | 58.522050059595 | | 1983 | 64.29512516469 | | 1982 | 57.671957671958 | | 1981 | 54.285714285714 | | 1980 | 63.157894736842 | | 1979 | 39.902280130293 | | 1978 | 40.569395017794 | | 1977 | 37.122557726465 | | 1976 | 32.298136645963 | | 1975 | 18.078175895766 | | 1974 | 13.120567375887 |
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