| Year | Value | | 2021 | 4.0809362099937 | | 2020 | 4.1324421196035 | | 2019 | 4.184789445263 | | 2018 | 4.2355739613214 | | 2017 | 4.2869655643882 | | 2016 | 4.3457162492064 | | 2015 | 4.3972997650344 | | 2014 | 4.457277106057 | | 2013 | 4.5275655016853 | | 2012 | 4.5868855773454 | | 2011 | 4.6575450922145 | | 2010 | 4.7278183178 | | 2009 | 4.8015919017145 | | 2008 | 4.8604039178821 | | 2007 | 4.8919532444426 | | 2006 | 4.8905043844835 | | 2005 | 4.9370304674192 | | 2004 | 5.0063547102732 | | 2003 | 5.0715949508998 | | 2002 | 5.1537952528778 | | 2001 | 5.2064141619625 | | 2000 | 5.2739789793056 | | 1999 | 5.3359529330485 | | 1998 | 5.393610595642 | | 1997 | 5.4963594524972 | | 1996 | 5.603507372136 | | 1995 | 5.7052072518409 | | 1994 | 5.7792049000857 | | 1993 | 5.8730865108166 | | 1992 | 5.9658992228748 | | 1991 | 6.0518592935182 | | 1990 | 6.1072159093305 | | 1989 | 6.1769177755797 | | 1988 | 6.2315200807274 | | 1987 | 6.2918098627294 | | 1986 | 6.3580512987219 | | 1985 | 6.4090592749919 | | 1984 | 6.471332223254 | | 1983 | 6.5272191168175 | | 1982 | 6.5703628678463 | | 1981 | 6.6103571421938 | | 1980 | 6.6525235873687 | | 1979 | 6.6957867815981 | | 1978 | 6.7334502043196 | | 1977 | 6.743289862878 | | 1976 | 6.7474439759994 | | 1975 | 6.7526258503241 | | 1974 | 6.7405878493549 | | 1973 | 6.7261600404992 | | 1972 | 6.7151890101503 | | 1971 | 6.7030843404129 | | 1970 | 6.682222007385 | | 1969 | 6.6725152778904 | | 1968 | 6.6680072272878 | | 1967 | 6.6710187025444 | | 1966 | 6.6706083625433 | | 1965 | 6.6682987622822 | | 1964 | 6.6560806419946 | | 1963 | 6.6415924237764 | | 1962 | 6.6411272226747 | | 1961 | 6.6214156472374 | | 1960 | 6.6070806494519 |
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