| Year | Value | | 2022 | 4.434 | | 2021 | 5.043 | | 2020 | 5.637 | | 2019 | 5.018 | | 2018 | 5.131 | | 2017 | 5.833 | | 2016 | 5.989 | | 2015 | 6.278 | | 2014 | 6.925 | | 2013 | 7.384 | | 2012 | 7.798 | | 2011 | 7.769 | | 2010 | 7.747 | | 2009 | 6.409 | | 2008 | 3.683 | | 2007 | 3.801 | | 2006 | 3.897 | | 2005 | 4.83 | | 2004 | 5.204 | | 2003 | 4.19 | | 2002 | 4.274 | | 2001 | 4.164 | | 2000 | 4.476 | | 1999 | 5.142 | | 1998 | 5.039 | | 1997 | 5.4 | | 1996 | 6.844 | | 1995 | 6.992 | | 1994 | 8.036 | | 1993 | 10.719 | | 1992 | 9.027 | | 1991 | 8.89 | | 1990 | 8.338 | | 1989 | 8.149 | | 1988 | 6.486 | | 1987 | 6.091 | | 1986 | 6.033 | | 1985 | 7.797 | | 1984 | 8.868 | | 1983 | 9.745 | | 1982 | 10 | | 1981 | 9.2 | | 1980 | 7 | | 1979 | 6.1 | | 1978 | 7.3 | | 1977 | 6.4 | | 1976 | 5.3 | | 1975 | 5.1 | | 1974 | 2.1 | | 1973 | 0.9 |
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