| Year | Value | | 2021 | 3.312494155 | | 2020 | 2.573077 | | 2019 | 2.091018 | | 2017 | 0.041650904321273 | | 2016 | 0.133149772 | | 2015 | 3.0955234848327 | | 2014 | 3.578448 | | 2013 | 4.47306 | | 2012 | 4.5788317676435 | | 2011 | 3.3757805514281 | | 2001 | 7.364 | | 2000 | 12.273 | | 1999 | 13.6999998093 | | 1998 | 13.5 | | 1997 | 15.3000001907 | | 1996 | 16.5 | | 1995 | 15.3000001907 | | 1994 | 14.5 | | 1993 | 14.3000001907 | | 1992 | 15.1999998093 | | 1991 | 16.1000003815 | | 1990 | 17.7000007629 | | 1989 | 18 | | 1988 | 15.8999996185 | | 1987 | 16.2999992371 | | 1986 | 16.5 | | 1985 | 18.3999996185 | | 1984 | 17.3999996185 | | 1983 | 18.7000007629 | | 1982 | 21 | | 1981 | 21 | | 1980 | 18 | | 1979 | 17.2000007629 | | 1978 | 14.6999998093 | | 1977 | 13.1000003815 | | 1976 | 13.1999998093 | | 1975 | 11.6000003815 | | 1974 | 10.8000001907 | | 1973 | 8.3000001907 | | 1972 | 8 | | 1971 | 7.0999999046 | | 1970 | 5.8000001907 |
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