| Year | Value | | 2020 | 3.555 | | 2019 | 3.518 | | 2018 | 2.57 | | 2004 | 2.7 | | 2000 | 2.588 | | 1995 | 2.434 | | 1993 | 1.9 | | 1992 | 1.9 | | 1991 | 1.9 | | 1990 | 1.8 | | 1989 | 1.8 | | 1988 | 1.8 | | 1987 | 1.8 | | 1986 | 1.7 | | 1983 | 1.6 | | 1982 | 1.6 | | 1981 | 1.5 | | 1980 | 1.5 | | 1979 | 1.4 | | 1978 | 1.4 | | 1977 | 1.3 | | 1976 | 1.3 | | 1975 | 1.3 | | 1970 | 1.2 | | 1965 | 1.2 | | 1960 | 1.1 |
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