| Year | Value | | 2021 | 4.624590777856 | | 2020 | 4.6959062249746 | | 2019 | 4.7662455102766 | | 2018 | 4.8261232329273 | | 2017 | 4.8880829997506 | | 2016 | 4.9572331615618 | | 2015 | 5.0223935208967 | | 2014 | 5.086390409653 | | 2013 | 5.1558686460188 | | 2012 | 5.227104301789 | | 2011 | 5.3094523348503 | | 2010 | 5.3945592417789 | | 2009 | 5.4799863808934 | | 2008 | 5.5489616704534 | | 2007 | 5.616005742948 | | 2006 | 5.6844818790567 | | 2005 | 5.7462901885275 | | 2004 | 5.8105802643297 | | 2003 | 5.871330280903 | | 2002 | 5.9338338509328 | | 2001 | 5.9917260221708 | | 2000 | 6.0432432374962 | | 1999 | 6.0998761198052 | | 1998 | 6.1511480631756 | | 1997 | 6.2032801311338 | | 1996 | 6.2522028387723 | | 1995 | 6.313946160659 | | 1994 | 6.3520766631094 | | 1993 | 6.388104615817 | | 1992 | 6.4219667418207 | | 1991 | 6.4575567347439 | | 1990 | 6.4958812269379 | | 1989 | 6.5345024393504 | | 1988 | 6.5721178095513 | | 1987 | 6.6104229564754 | | 1986 | 6.6319889656509 | | 1985 | 6.6445172469024 | | 1984 | 6.6535272561089 | | 1983 | 6.6592504081756 | | 1982 | 6.6700180289239 | | 1981 | 6.6939400684682 | | 1980 | 6.7065363075423 | | 1979 | 6.7162635563896 | | 1978 | 6.706826969322 | | 1977 | 6.6952352437767 | | 1976 | 6.6852134107934 | | 1975 | 6.6950123498718 | | 1974 | 6.694483986721 | | 1973 | 6.7050690917921 | | 1972 | 6.7521742518388 | | 1971 | 6.720068807286 | | 1970 | 6.7103605676744 | | 1969 | 6.6909118971347 | | 1968 | 6.6691203450478 | | 1967 | 6.6588275630569 | | 1966 | 6.5939347301153 | | 1965 | 6.5507762054665 | | 1964 | 6.5306614353897 | | 1963 | 6.5215109629516 | | 1962 | 6.4821902316603 | | 1961 | 6.4491688744133 | | 1960 | 6.4720315696504 |
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