| Year | Value | | 2022 | 2.7580323433586 | | 2021 | 2.8408059778949 | | 2020 | 2.9338181351684 | | 2019 | 3.0076181068364 | | 2018 | 3.0618878016671 | | 2017 | 3.1135954930269 | | 2016 | 3.1474074931083 | | 2015 | 3.1918962618366 | | 2014 | 3.2471180031464 | | 2013 | 3.2712989421351 | | 2012 | 3.3014801916672 | | 2011 | 3.3771096377685 | | 2010 | 3.4971913221253 | | 2009 | 3.5548434351805 | | 2008 | 3.571096576992 | | 2007 | 3.5329214446065 | | 2006 | 3.4562366804267 | | 2005 | 3.3126704214899 | | 2004 | 3.1789364686254 | | 2003 | 3.0891135791469 | | 2002 | 3.056528347334 | | 2001 | 2.9960564104668 | | 2000 | 2.7666055910369 | | 1999 | 2.6200545684698 | | 1998 | 2.5861092462704 | | 1997 | 2.5633203535811 | | 1996 | 2.4780307216592 | | 1995 | 2.4469656113813 | | 1994 | 2.428275601566 | | 1993 | 2.4054625122411 | | 1992 | 2.4303721297642 | | 1991 | 2.4934375613718 | | 1990 | 2.5713653551927 | | 1989 | 2.664236144733 | | 1988 | 2.7680263591564 | | 1987 | 2.9204961463984 | | 1986 | 3.0128482515706 | | 1985 | 3.011752683073 | | 1984 | 3.0834246629317 | | 1983 | 3.2348776275938 | | 1982 | 3.2257080666493 | | 1981 | 3.0479934703707 | | 1980 | 2.9623858816794 | | 1979 | 2.9624772276847 | | 1978 | 2.9756219227253 | | 1977 | 2.9793832989068 | | 1976 | 2.9654389753036 | | 1975 | 2.9262746643996 | | 1974 | 2.8755274779589 | | 1973 | 2.8270344588423 | | 1972 | 2.7717805147822 | | 1971 | 2.724277770368 | | 1970 | 2.9067010103921 | | 1969 | 3.171519987442 | | 1968 | 3.2254388958623 | | 1967 | 3.2196690431166 | | 1966 | 3.2186953682621 | | 1965 | 3.2015897461979 | | 1964 | 3.1944409394148 | | 1963 | 3.1966324591177 | | 1962 | 3.1785631336462 | | 1961 | 3.1560555302233 |
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