| Year | Value | | 2021 | 4.6014625918666 | | 2020 | 4.6670234914749 | | 2019 | 4.7344582595024 | | 2018 | 4.7881921253311 | | 2017 | 4.8412145188941 | | 2016 | 4.8974034951071 | | 2015 | 4.957176100739 | | 2014 | 5.0157720803961 | | 2013 | 5.0849237250002 | | 2012 | 5.157179483145 | | 2011 | 5.233563768474 | | 2010 | 5.3022933837401 | | 2009 | 5.3668952614615 | | 2008 | 5.4293822228973 | | 2007 | 5.4596288368306 | | 2006 | 5.4975068978484 | | 2005 | 5.5289393344613 | | 2004 | 5.5597520836792 | | 2003 | 5.6000407874917 | | 2002 | 5.6439080361656 | | 2001 | 5.686088240007 | | 2000 | 5.7220605583283 | | 1999 | 5.7627036135711 | | 1998 | 5.8021040666329 | | 1997 | 5.8597123322405 | | 1996 | 5.9303238803357 | | 1995 | 5.9981872842527 | | 1994 | 6.0538908746451 | | 1993 | 6.1172666389379 | | 1992 | 6.1815780963812 | | 1991 | 6.244767805217 | | 1990 | 6.3040175791041 | | 1989 | 6.3816269161982 | | 1988 | 6.4442282849722 | | 1987 | 6.5048582530012 | | 1986 | 6.5502066033724 | | 1985 | 6.5982488263176 | | 1984 | 6.6511337007279 | | 1983 | 6.6949396172694 | | 1982 | 6.7262934481777 | | 1981 | 6.7564774921564 | | 1980 | 6.7827005515072 | | 1979 | 6.8050611655965 | | 1978 | 6.8208491637824 | | 1977 | 6.8203972928531 | | 1976 | 6.8172227241579 | | 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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