| Year | Value | | 2022 | 1.5025049767048 | | 2021 | 1.5145516432228 | | 2020 | 1.6164266520424 | | 2019 | 1.7280333995771 | | 2018 | 1.7282415040642 | | 2017 | 1.7018067191303 | | 2016 | 1.8292821910401 | | 2015 | 1.9907906851906 | | 2014 | 2.0495242048307 | | 2013 | 1.9847598578577 | | 2012 | 1.8496603492438 | | 2011 | 1.880740255556 | | 2010 | 1.9717211763461 | | 2009 | 1.9321411977519 | | 2008 | 1.9170897571612 | | 2007 | 1.9175002231094 | | 2006 | 1.9344512276387 | | 2005 | 1.9620447126285 | | 2004 | 1.9469605583112 | | 2003 | 1.7939792973124 | | 2002 | 1.8200710264898 | | 2001 | 1.988373616247 | | 2000 | 1.9910588803486 | | 1999 | 1.9850554596523 | | 1998 | 2.0077993944171 | | 1997 | 2.0327691980131 | | 1996 | 2.0557394617822 | | 1995 | 2.1422629329582 | | 1994 | 2.0917476879129 | | 1993 | 2.1887703043044 | | 1992 | 2.5032392363671 | | 1991 | 2.7954812250257 | | 1990 | 3.7657909754084 | | 1989 | 2.7717287510938 | | 1988 | 2.8299149967268 | | 1987 | 2.977532042779 | | 1986 | 3.0575997611811 | | 1985 | 3.1041077520309 | | 1984 | 3.1860078196917 | | 1983 | 3.1439013046766 | | 1982 | 3.423380854044 | | 1981 | 3.4490401078202 | | 1980 | 3.081610142926 | | 1979 | 2.7787123195615 | | 1978 | 2.707237949931 | | 1977 | 3.173248355352 | | 1976 | 3.2443185171814 | | 1975 | 2.8880836916806 | | 1974 | 2.7136418492678 | | 1973 | 2.6808416126887 | | 1972 | 2.6646315505467 | | 1971 | 2.6850697378686 | | 1970 | 2.7183850067803 | | 1969 | 2.7288465052337 | | 1968 | 2.7363819599705 | | 1967 | 2.7406845801044 | | 1966 | 2.6968304803114 | | 1965 | 2.6536170090934 | | 1964 | 2.6325416444833 | | 1963 | 2.6018095417263 | | 1962 | 2.5492940519092 | | 1961 | 2.5114608349715 |
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