| Year | Value | | 2021 | 5.5454648674699 | | 2020 | 5.285240173373 | | 2019 | 4.7936485414789 | | 2018 | 4.7730463790142 | | 2017 | 4.7785776036164 | | 2016 | 4.8980857086921 | | 2015 | 4.9560751557174 | | 2014 | 5.0072481095559 | | 2013 | 5.0576093638988 | | 2012 | 5.0104190559559 | | 2011 | 4.9154492031398 | | 2010 | 4.9568575450879 | | 2009 | 5.0053862329592 | | 2008 | 5.0902596381255 | | 2007 | 5.1536997324703 | | 2006 | 5.2354414740739 | | 2005 | 5.2675924731869 | | 2004 | 5.3191698458146 | | 2003 | 5.4470984102793 | | 2002 | 5.3431805878022 | | 2001 | 5.4269800611981 | | 2000 | 5.5187844601639 | | 1999 | 5.6197477923003 | | 1998 | 5.7520971768127 | | 1997 | 5.7994096091435 | | 1996 | 5.8805547752538 | | 1995 | 6.0428464806709 | | 1994 | 6.1944759994793 | | 1993 | 6.2982436565274 | | 1992 | 6.4530739462943 | | 1991 | 6.7225767466757 | | 1990 | 7.2699539795628 | | 1989 | 7.34267544464 | | 1988 | 7.7517235338182 | | 1987 | 7.9839963603675 | | 1986 | 8.3838292412026 | | 1985 | 8.7338577250987 | | 1984 | 9.1928228616793 | | 1983 | 9.8626068602115 | | 1982 | 10.695271533214 | | 1981 | 11.030130175481 | | 1980 | 11.182048017714 | | 1979 | 11.537901751004 | | 1978 | 12.240724798858 | | 1977 | 12.631430049706 | | 1976 | 13.520620619835 | | 1975 | 13.697587726811 | | 1974 | 14.158036450555 | | 1973 | 14.737099645278 | | 1972 | 15.252575500787 | | 1971 | 15.738919260323 | | 1970 | 16.288167943961 | | 1969 | 16.787425215637 | | 1968 | 17.321255237862 | | 1967 | 17.7315316583 | | 1966 | 18.186569230978 | | 1965 | 18.646289534679 | | 1964 | 19.163227579537 | | 1963 | 19.727297840747 | | 1962 | 20.596623383715 | | 1961 | 20.823806557439 | | 1960 | 21.470191984863 |
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