| Year | Value | | 2022 | 3.0775014574771 | | 2021 | 3.7676423078602 | | 2020 | 4.3573923998606 | | 2019 | 3.4977415172529 | | 2018 | 3.6700895885476 | | 2017 | 3.6528881386636 | | 2016 | 3.8033191717505 | | 2015 | 3.0635768188228 | | 2014 | 2.6497630061104 | | 2013 | 2.422453044963 | | 2012 | 2.2438589953323 | | 2011 | 2.0215965340095 | | 2010 | 2.1520110520818 | | 2009 | 2.25932565825 | | 2008 | 2.1256404060779 | | 2007 | 2.1558935651696 | | 2006 | 2.0357854180951 | | 2005 | 2.4217662131311 | | 2004 | 2.6820855576706 | | 2003 | 2.8556788194 | | 2002 | 2.9270921635444 | | 2001 | 2.6193522704655 | | 2000 | 2.4899229886941 | | 1999 | 2.5907962519192 | | 1998 | 2.7767386109436 | | 1997 | 2.6405581380412 | | 1996 | 2.7001331051531 | | 1995 | 3.3111257255597 | | 1994 | 3.455478314892 | | 1993 | 3.7528588650165 | | 1992 | 3.7761117152178 | | 1991 | 3.5789382878463 | | 1990 | 4.3569744640536 | | 1989 | 4.4991785881309 | | 1988 | 4.8379577211514 | | 1987 | 4.8699393702327 | | 1986 | 5.5778382053025 | | 1985 | 5.525664986502 | | 1984 | 5.1868513651953 | | 1983 | 4.9500376844716 | | 1982 | 5.4266220594821 | | 1981 | 5.5015290519878 | | 1980 | 5.5972050367567 | | 1979 | 5.8056821570047 | | 1978 | 5.2803433156078 | | 1977 | 5.8314350797267 | | 1976 | 5.5082742316785 | | 1975 | 5.9384267758791 | | 1974 | 5.5412844036697 | | 1973 | 6.0620356174077 | | 1972 | 5.9008555611475 |
|