| Year | Value | | 2021 | 4.028365 | | 2020 | 4.317833 | | 2019 | 9.166862 | | 2018 | 4.1602689844241 | | 2017 | 5.4194066666667 | | 2016 | 1.2022303382588 | | 2015 | 0.98336807284283 | | 2014 | 4.71912 | | 2013 | 4.563948 | | 2011 | 2.708603616984 | | 2010 | 15.376963961 | | 2001 | 7.364 | | 2000 | 12.292 | | 1999 | 13.8000001907 | | 1998 | 13.6000003815 | | 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 | 18.1000003815 | | 1988 | 15.8999996185 | | 1987 | 16.2999992371 | | 1986 | 16.6000003815 | | 1985 | 18.3999996185 | | 1984 | 17.7999992371 | | 1983 | 19.1000003815 | | 1982 | 21.3999996185 | | 1981 | 21.3999996185 | | 1980 | 18.2000007629 | | 1979 | 17.2999992371 | | 1978 | 14.8000001907 | | 1977 | 13.1999998093 | | 1976 | 13.1999998093 | | 1975 | 11.6000003815 | | 1974 | 10.8999996185 | | 1973 | 8.3999996185 | | 1972 | 8.1000003815 | | 1971 | 7.1999998093 | | 1970 | 5.9000000954 |
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