| Year | Value | | 2021 | 4.8441712610976 | | 2020 | 4.9162762045239 | | 2019 | 4.9886842219481 | | 2018 | 5.0629107749614 | | 2017 | 5.139025707974 | | 2016 | 5.2132303818432 | | 2015 | 5.283577660475 | | 2014 | 5.3553111719745 | | 2013 | 5.4329907808021 | | 2012 | 5.5115281668722 | | 2011 | 5.5958273358698 | | 2010 | 5.6632128579444 | | 2009 | 5.7226073264047 | | 2008 | 5.7709574429651 | | 2007 | 5.8106217784844 | | 2006 | 5.8519938940832 | | 2005 | 5.8838691240484 | | 2004 | 5.9259075227318 | | 2003 | 5.9723050974667 | | 2002 | 6.0181743858528 | | 2001 | 6.0615544814364 | | 2000 | 6.0996251886012 | | 1999 | 6.1348897412044 | | 1998 | 6.1704773219032 | | 1997 | 6.2229903872149 | | 1996 | 6.2723558970998 | | 1995 | 6.3227516550864 | | 1994 | 6.3697583023848 | | 1993 | 6.422840647173 | | 1992 | 6.4762193411152 | | 1991 | 6.5267442098212 | | 1990 | 6.5785583023998 | | 1989 | 6.6319373982458 | | 1988 | 6.6736219426322 | | 1987 | 6.7158169511065 | | 1986 | 6.7553905281939 | | 1985 | 6.7904628920449 | | 1984 | 6.8303050222376 | | 1983 | 6.8662422279414 | | 1982 | 6.8887244566625 | | 1981 | 6.9189937875874 | | 1980 | 6.9447609358781 | | 1979 | 6.9698907624244 | | 1978 | 6.9850253797994 | | 1977 | 6.9782073551834 | | 1976 | 6.9670544680157 | | 1975 | 6.9626247748858 | | 1974 | 6.9452286726438 | | 1973 | 6.9240564718652 | | 1972 | 6.9102160026003 | | 1971 | 6.8914412525982 | | 1970 | 6.8622162718643 | | 1969 | 6.8430189805971 | | 1968 | 6.8217870908018 | | 1967 | 6.7946901288362 | | 1966 | 6.7702646367826 | | 1965 | 6.7421014935961 | | 1964 | 6.7164049286552 | | 1963 | 6.6908853017183 | | 1962 | 6.6680439688907 | | 1961 | 6.6027478865143 | | 1960 | 6.5636448376619 |
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