| Year | Value | | 2019 | 4.1511087434015 | | 2018 | 4.5971606189804 | | 2017 | 4.5971606189804 | | 2016 | 4.5241108988037 | | 2015 | 4.3669393732962 | | 2014 | 1.992936 | | 2013 | 2.384004 | | 2012 | 1.5978152047995 | | 2011 | 1.4348805835051 | | 2010 | 1.8665736803563 | | 2001 | 7.364 | | 2000 | 12.273 | | 1999 | 13.6999998093 | | 1998 | 13.8000001907 | | 1997 | 15.6000003815 | | 1996 | 17.2000007629 | | 1995 | 16 | | 1994 | 15.1000003815 | | 1993 | 14.8999996185 | | 1992 | 15.8000001907 | | 1991 | 16.7999992371 | | 1990 | 18.3999996185 | | 1989 | 18.6000003815 | | 1988 | 16.5 | | 1987 | 16.8999996185 | | 1986 | 17.7000007629 | | 1985 | 20 | | 1984 | 19 | | 1983 | 20.2000007629 | | 1982 | 21.5 | | 1981 | 21.3999996185 | | 1980 | 18.3999996185 | | 1979 | 17.5 | | 1978 | 15.1000003815 | | 1977 | 13.3999996185 | | 1976 | 13.5 | | 1975 | 11.8000001907 | | 1974 | 11.1000003815 | | 1973 | 8.5 | | 1972 | 8.1999998093 | | 1971 | 7.3000001907 | | 1970 | 5.9000000954 |
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