| Year | Value | | 2022 | 3.0947670449434 | | 2021 | 2.4019430906139 | | 2020 | 2.2013151236168 | | 2019 | 2.2384758047347 | | 2018 | 1.9050221351764 | | 2017 | 1.3636298711596 | | 2016 | 1.1653328946816 | | 2015 | 1.2507807015299 | | 2014 | 1.272342619383 | | 2013 | 1.2360893348902 | | 2012 | 1.1760292310267 | | 2011 | 1.150905295468 | | 2010 | 1.2247855431023 | | 2009 | 1.3527649759801 | | 2008 | 1.2429126083038 | | 2007 | 1.4157265281638 | | 2006 | 1.1103217971972 | | 2005 | 1.2018948481342 | | 2004 | 1.2070057657012 | | 2003 | 1.0730553376671 | | 2002 | 1.1779349770001 | | 2001 | 1.1555150239697 | | 2000 | 1.2380463806297 | | 1999 | 1.3855313863293 | | 1998 | 1.2763626403725 | | 1997 | 1.417055044716 | | 1996 | 1.287088470397 | | 1995 | 1.3830426939266 | | 1994 | 1.4228847293978 | | 1993 | 1.8625298848076 | | 1992 | 2.0496515679443 | | 1991 | 2.1668692673224 | | 1990 | 2.7048929663609 | | 1989 | 2.7407740774077 | | 1988 | 2.310812644146 | | 1987 | 2.2525864617204 | | 1986 | 2.6218934911243 | | 1985 | 1.8365254109486 | | 1984 | 3.0171674369183 | | 1983 | 2.9321834163139 | | 1982 | 3.0033036339974 | | 1981 | 2.8426896977175 | | 1980 | 2.7466911764706 | | 1979 | 2.6986138613861 | | 1978 | 3.2831460674157 | | 1977 | 3.0432666306111 | | 1976 | 3.123828647925 | | 1975 | 2.8111837327524 | | 1974 | 1.1519083969466 | | 1973 | 1.2351868732908 | | 1972 | 1.1398537477148 | | 1971 | 1.1771653543307 | | 1970 | 1.1722391084093 | | 1969 | 1.2467222884386 | | 1968 | 1.1784810126582 | | 1967 | 1.3250728862974 | | 1966 | 1.6494845360825 | | 1965 | 1.5248226950355 | | 1964 | 2.6051587301587 | | 1963 | 2.3830570902394 | | 1962 | 2.345703125 | | 1961 | 0.8343685300207 | | 1960 | 0.68201754385965 |
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