| Year | Value | | 2021 | 21.945137157107 | | 2020 | 21.945137157107 | | 2019 | 21.945137157107 | | 2018 | 21.945137157107 | | 2017 | 21.945137157107 | | 2016 | 21.945137157107 | | 2015 | 21.945137157107 | | 2014 | 21.945137157107 | | 2013 | 21.940288168468 | | 2012 | 20.388611803824 | | 2011 | 22.895954558049 | | 2010 | 21.894984760321 | | 2009 | 20.240648379052 | | 2008 | 19.011221945137 | | 2007 | 17.784981989471 | | 2006 | 23.412025491826 | | 2005 | 20.393737877528 | | 2004 | 19.918675533389 | | 2003 | 17.133139373788 | | 2002 | 12.608478802993 | | 2001 | 9.4419506788584 | | 2000 | 6.7885840953173 | | 1999 | 6.7054585757828 | | 1998 | 6.7054585757828 | | 1997 | 6.7054585757828 | | 1996 | 6.7193128290385 | | 1995 | 6.7193128290385 | | 1994 | 6.7193128290385 | | 1993 | 6.7331670822943 | | 1992 | 6.7331670822943 | | 1991 | 6.7331670822943 | | 1990 | 6.7331670822943 | | 1989 | 6.7331670822943 | | 1988 | 6.7331670822943 | | 1987 | 6.7193128290385 | | 1986 | 6.6500415627598 | | 1985 | 6.580770296481 | | 1984 | 6.5114990302023 | | 1983 | 6.4422277639235 | | 1982 | 6.3729564976448 | | 1981 | 6.303685231366 | | 1980 | 6.2344139650873 | | 1979 | 6.1651426988085 | | 1978 | 6.0958714325298 | | 1977 | 6.026600166251 | | 1976 | 5.9573288999723 | | 1975 | 5.8880576336935 | | 1974 | 5.8187863674148 | | 1973 | 5.7495151011361 | | 1972 | 5.6802438348573 | | 1971 | 5.6109725685785 | | 1970 | 5.5417013022998 | | 1969 | 5.4724300360211 | | 1968 | 5.4031587697423 | | 1967 | 5.3338875034636 | | 1966 | 5.2646162371848 | | 1965 | 5.1953449709061 | | 1964 | 5.1260737046273 | | 1963 | 5.0568024383486 | | 1962 | 4.9875311720698 | | 1961 | 4.9182599057911 |
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