| Year | Value | | 2021 | 4.6012882827853 | | 2020 | 4.6668242773245 | | 2019 | 4.7342529805052 | | 2018 | 4.7879797395744 | | 2017 | 4.8409903831632 | | 2016 | 4.8971652373664 | | 2015 | 4.9569170641054 | | 2014 | 5.015495964933 | | 2013 | 5.084633870895 | | 2012 | 5.1568862748684 | | 2011 | 5.2332488620626 | | 2010 | 5.3019303789705 | | 2009 | 5.3665478218802 | | 2008 | 5.4290109111659 | | 2007 | 5.459238507725 | | 2006 | 5.4970843954023 | | 2005 | 5.5285194778702 | | 2004 | 5.5592926775477 | | 2003 | 5.5995677426863 | | 2002 | 5.6434081981965 | | 2001 | 5.6855777309845 | | 2000 | 5.7215472501124 | | 1999 | 5.7621715772866 | | 1998 | 5.8015654292938 | | 1997 | 5.8591741544048 | | 1996 | 5.9303238803357 | | 1995 | 5.9981872842527 | | 1994 | 6.0538908746451 | | 1993 | 6.1172666389379 | | 1992 | 6.1810808120626 | | 1991 | 6.244767805217 | | 1990 | 6.3040175791041 | | 1989 | 6.3816269161982 | | 1988 | 6.4442282849722 | | 1987 | 6.5043274082047 | | 1986 | 6.5502066033724 | | 1985 | 6.5982488263176 | | 1984 | 6.6511337007279 | | 1983 | 6.6949396172694 | | 1982 | 6.7257786383452 | | 1981 | 6.7564774921564 | | 1980 | 6.7827005515072 | | 1979 | 6.8050611655965 | | 1978 | 6.8208491637824 | | 1977 | 6.8203972928531 | | 1976 | 6.8168451305668 | | 1975 | 6.8196933533239 | | 1974 | 6.8134150008838 | | 1973 | 6.8041766515989 | | 1972 | 6.7965609869501 | | 1971 | 6.7838611473281 | | 1970 | 6.762868406369 | | 1969 | 6.7511890483393 | | 1968 | 6.7380883025937 | | 1967 | 6.7223233297566 | | 1966 | 6.7068276402207 | | 1965 | 6.690410452499 | | 1964 | 6.6763356775862 | | 1963 | 6.6619613510463 | | 1962 | 6.646363845269 | | 1961 | 6.6257040438326 | | 1960 | 6.6090962456072 |
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