| Year | Value | | 2022 | 2.6306142607878 | | 2021 | 2.7010459399939 | | 2020 | 2.7567787845457 | | 2019 | 2.8935899689309 | | 2018 | 3.191487022098 | | 2017 | 3.2546161706573 | | 2016 | 3.1105453128546 | | 2015 | 3.1075116713104 | | 2014 | 2.7744928612481 | | 2013 | 2.3357525902736 | | 2012 | 2.127629014379 | | 2011 | 1.9944775499975 | | 2010 | 2.3746825891688 | | 2009 | 2.716265689486 | | 2008 | 2.7637791039017 | | 2007 | 2.7899643696556 | | 2006 | 2.6470364070866 | | 2005 | 2.4294536103333 | | 2004 | 2.2539258642775 | | 2003 | 2.219125575762 | | 2002 | 2.2858396584804 | | 2001 | 2.4359078557777 | | 2000 | 2.5596369083723 | | 1999 | 2.3885009121631 | | 1998 | 2.3231892056128 | | 1997 | 2.4510966276376 | | 1996 | 2.4248115999504 | | 1995 | 2.5525063922726 | | 1994 | 2.4089310747889 | | 1993 | 1.7440730486617 | | 1992 | 1.5097189005351 | | 1991 | 1.7056633363295 | | 1990 | 1.7803895539437 | | 1989 | 1.2867495280515 | | 1988 | 1.0976664808068 | | 1987 | 1.7103968327108 | | 1986 | 1.8793056958959 | | 1985 | 1.8166551883901 | | 1984 | 2.2789583349279 | | 1983 | 3.2847276180847 | | 1982 | 4.0773141132874 | | 1981 | 4.2893579973797 | | 1980 | 4.2923865207415 | | 1979 | 4.2669691858214 | | 1978 | 4.2617298086617 | | 1977 | 4.1853899913102 | | 1976 | 4.1253366659246 | | 1975 | 4.1337597779588 | | 1974 | 3.7877110663651 | | 1973 | 3.3625992297923 | | 1972 | 3.2737985412715 | | 1971 | 3.166565513693 | | 1970 | 3.045052800682 | | 1969 | 3.0295798467018 | | 1968 | 3.0384792623554 | | 1967 | 3.00976664571 | | 1966 | 3.0591483142482 | | 1965 | 3.2020644941814 | | 1964 | 3.1491022326379 | | 1963 | 3.0493168962171 | | 1962 | 3.0296892753304 | | 1961 | 2.9702693845419 |
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