| Year | Value | | 2022 | 2.4527032008119 | | 2021 | 2.455303010444 | | 2020 | 2.5086182033488 | | 2019 | 2.531287445603 | | 2018 | 2.5664749340504 | | 2017 | 2.5862542961096 | | 2016 | 2.5806758084553 | | 2015 | 2.5807672893183 | | 2014 | 2.3152427484718 | | 2013 | 2.0619264950119 | | 2012 | 2.0554349340197 | | 2011 | 2.0752447134814 | | 2010 | 2.1163624191381 | | 2009 | 2.117819076401 | | 2008 | 2.1305154748482 | | 2007 | 2.1608798837222 | | 2006 | 2.2096561363888 | | 2005 | 2.2675137159944 | | 2004 | 2.3374559903094 | | 2003 | 2.4165969031491 | | 2002 | 2.510506715141 | | 2001 | 2.6190140962377 | | 2000 | 2.7382509053151 | | 1999 | 3.0240202346513 | | 1998 | 3.3569194562211 | | 1997 | 3.4800835397912 | | 1996 | 3.5126055824823 | | 1995 | 3.540628800804 | | 1994 | 3.5834195282798 | | 1993 | 3.6565981654803 | | 1992 | 3.7218891154323 | | 1991 | 3.7791367684815 | | 1990 | 3.8304237789531 | | 1989 | 3.730526455312 | | 1988 | 3.4987712547295 | | 1987 | 3.3766527143988 | | 1986 | 3.4073855886668 | | 1985 | 3.499582055974 | | 1984 | 3.5907876334849 | | 1983 | 3.6639084973874 | | 1982 | 3.717022456039 | | 1981 | 3.7558672418034 | | 1980 | 3.7727007031637 | | 1979 | 3.7661141319871 | | 1978 | 3.7699953456267 | | 1977 | 3.7536887715446 | | 1976 | 4.1259804754355 | | 1975 | 4.552904269813 | | 1974 | 4.5540160173957 | | 1973 | 4.534255609893 | | 1972 | 4.4647183524872 | | 1971 | 4.3197501143839 | | 1970 | 4.1364849829396 | | 1969 | 3.979912470958 | | 1968 | 3.9052303147511 | | 1967 | 3.8941788342667 | | 1966 | 3.8918994415869 | | 1965 | 3.8924128465564 | | 1964 | 3.8909952459826 | | 1963 | 3.8796046939883 | | 1962 | 3.8225172315895 | | 1961 | 3.6969952047495 |
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