| Year | Value | | 2022 | 1.3964956838661 | | 2021 | 1.4332140649594 | | 2020 | 1.4774566600306 | | 2019 | 1.4954048152646 | | 2018 | 1.518707268085 | | 2017 | 1.5343644950603 | | 2016 | 1.5198136905787 | | 2015 | 1.4239561649412 | | 2014 | 1.3649122882104 | | 2013 | 1.4026498609275 | | 2012 | 1.4309732746759 | | 2011 | 1.4585950847998 | | 2010 | 1.4894787970459 | | 2009 | 1.5214686167078 | | 2008 | 1.5523244760119 | | 2007 | 1.5808715339339 | | 2006 | 1.5869225795326 | | 2005 | 1.4594872312525 | | 2004 | 1.3808441145462 | | 2003 | 1.4690442270043 | | 2002 | 1.553068982121 | | 2001 | 1.6228567322206 | | 2000 | 1.6860072039373 | | 1999 | 1.7594520851929 | | 1998 | 1.8540443717705 | | 1997 | 2.0918807241896 | | 1996 | 2.3364408148927 | | 1995 | 2.4547162003654 | | 1994 | 2.584170378694 | | 1993 | 2.6863956080513 | | 1992 | 2.7535956926722 | | 1991 | 2.80609494356 | | 1990 | 2.8424893344121 | | 1989 | 2.8682162175613 | | 1988 | 2.869366890744 | | 1987 | 2.851262816833 | | 1986 | 2.8140482528778 | | 1985 | 2.754280282683 | | 1984 | 2.6754782908801 | | 1983 | 2.5773296526052 | | 1982 | 2.4236154514034 | | 1981 | 2.2036992181018 | | 1980 | 1.9543375060042 | | 1979 | 1.7392230990854 | | 1978 | 1.6563066478752 | | 1977 | 1.7337345879669 | | 1976 | 1.9491023928355 | | 1975 | 2.2095662961394 | | 1974 | 2.3804601887897 | | 1973 | 2.4367303162429 | | 1972 | 2.4350719178639 | | 1971 | 2.4169868860905 | | 1970 | 2.3896462461897 | | 1969 | 2.3645827680044 | | 1968 | 2.3456572246551 | | 1967 | 2.3166636039892 | | 1966 | 2.3017780587896 | | 1965 | 2.2931230266335 | | 1964 | 2.2743771194665 | | 1963 | 2.2962479789193 | | 1962 | 2.2981573061982 | | 1961 | 2.2643120165832 |
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