| Year | Value | | 2022 | 3.1978351045319 | | 2021 | 3.2225281599125 | | 2020 | 3.2244784432818 | | 2019 | 3.1862883889982 | | 2018 | 3.272827234455 | | 2017 | 3.4427669576009 | | 2016 | 3.4660346480854 | | 2015 | 3.3893913118086 | | 2014 | 3.4460167581905 | | 2013 | 3.4091449670035 | | 2012 | 3.3566561314375 | | 2011 | 3.3517939150768 | | 2010 | 3.2468806978207 | | 2009 | 3.1942467547909 | | 2008 | 3.1997436140875 | | 2007 | 3.2155379902249 | | 2006 | 3.1871715188242 | | 2005 | 3.1154617692253 | | 2004 | 2.9810565705433 | | 2003 | 2.9440511063098 | | 2002 | 3.0570002392583 | | 2001 | 3.0194658216436 | | 2000 | 2.8988412880662 | | 1999 | 2.8603206446613 | | 1998 | 2.3664088194401 | | 1997 | 1.5819574926089 | | 1996 | 1.90595041214 | | 1995 | 4.1842248005639 | | 1994 | 4.7636916876046 | | 1993 | 3.2052328862643 | | 1992 | 3.0123645797656 | | 1991 | 3.299649396561 | | 1990 | 3.2811690815344 | | 1989 | 3.1379146348683 | | 1988 | 3.1222428072175 | | 1987 | 3.0650355637621 | | 1986 | 2.9592828525983 | | 1985 | 3.0331938715881 | | 1984 | 2.9710144764096 | | 1983 | 2.7470991708152 | | 1982 | 2.7353609169095 | | 1981 | 2.7660830611367 | | 1980 | 3.0639304119175 | | 1979 | 3.4864127694439 | | 1978 | 3.0802260475511 | | 1977 | 2.5107953091945 | | 1976 | 2.5791635547733 | | 1975 | 2.6563539565351 | | 1974 | 2.6840340492344 | | 1973 | 2.6921780812841 | | 1972 | 2.66986839689 | | 1971 | 2.7468915428227 | | 1970 | 2.8622165784308 | | 1969 | 2.9029916699524 | | 1968 | 2.9376450858477 | | 1967 | 2.913202589288 | | 1966 | 2.8463431631942 | | 1965 | 2.7463503945878 | | 1964 | 2.6752402155853 | | 1963 | 2.6495842288037 | | 1962 | 2.5996943558265 | | 1961 | 2.5638110259069 |
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