| Year | Value | | 2021 | 5.5133976344692 | | 2020 | 5.4885947620389 | | 2019 | 5.4970430864545 | | 2018 | 5.3752464094621 | | 2017 | 5.4041115178823 | | 2016 | 5.4041115178823 | | 2015 | 5.3900309771895 | | 2014 | 5.3858068149817 | | 2013 | 5.3421571388341 | | 2012 | 5.3041396789637 | | 2011 | 5.4702900591383 | | 2010 | 5.5237961137708 | | 2009 | 5.4618417347226 | | 2008 | 7.5978597578147 | | 2007 | 7.4992959729654 | | 2006 | 7.5837792171219 | | 2005 | 7.8428611658688 | | 2004 | 7.8006195437905 | | 2003 | 7.8006195437905 | | 2002 | 7.9695860321036 | | 2001 | 7.9132638693326 | | 2000 | 7.7020557589411 | | 1999 | 7.2655589974655 | | 1998 | 5.6040551957195 | | 1997 | 5.5618135736412 | | 1996 | 5.4491692480991 | | 1995 | 5.3928470853281 | | 1994 | 5.2379611377077 | | 1993 | 4.9985919459307 | | 1992 | 4.7733032948465 | | 1991 | 4.3649676147564 | | 1990 | 4.0129540974373 | | 1989 | 3.3652492255703 | | 1988 | 3.3652492255703 | | 1987 | 3.3511686848775 | | 1986 | 3.3511686848775 | | 1985 | 3.3089270627992 | | 1984 | 3.2666854407209 | | 1983 | 3.2666854407209 | | 1982 | 3.1962827372571 | | 1981 | 3.1962827372571 | | 1980 | 3.1399605744861 | | 1979 | 3.083638411715 | | 1978 | 3.0695578710222 | | 1977 | 3.0554773303295 | | 1976 | 3.0413967896367 | | 1975 | 3.027316248944 | | 1974 | 3.0132357082512 | | 1973 | 2.9991551675584 | | 1972 | 2.9850746268657 | | 1971 | 2.9850746268657 | | 1970 | 2.9850746268657 | | 1969 | 2.9569135454802 | | 1968 | 2.9569135454802 | | 1967 | 2.9569135454802 | | 1966 | 2.9569135454802 | | 1965 | 2.9428330047874 | | 1964 | 2.9287524640946 | | 1963 | 2.9287524640946 | | 1962 | 2.9287524640946 | | 1961 | 2.9287524640946 |
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