| Year | Value | | 2022 | 2.7705662677782 | | 2021 | 2.7836312198382 | | 2020 | 2.8371827907695 | | 2019 | 2.770123730062 | | 2018 | 2.8070624476027 | | 2017 | 2.9726172819343 | | 2016 | 3.0975646302674 | | 2015 | 3.0467126593569 | | 2014 | 3.0478603847771 | | 2013 | 3.0384911488364 | | 2012 | 3.108166190083 | | 2011 | 3.1938745685078 | | 2010 | 3.3813984398439 | | 2009 | 3.8824233474802 | | 2008 | 3.850026794032 | | 2007 | 3.602552791077 | | 2006 | 3.9229111936544 | | 2005 | 4.2750431312082 | | 2004 | 4.4168050507379 | | 2003 | 4.8696476480481 | | 2002 | 4.9249862874241 | | 2001 | 4.8262499992186 | | 2000 | 4.5188862936598 | | 1999 | 5.2073539879343 | | 1998 | 5.2110529607091 | | 1997 | 4.4500364770867 | | 1996 | 4.2533140389894 | | 1995 | 4.1787978836896 | | 1994 | 3.7914351280791 | | 1993 | 4.091764933998 | | 1992 | 4.4713686460427 | | 1991 | 4.4483566730442 | | 1990 | 4.6321444729836 | | 1989 | 4.4928982562508 | | 1988 | 4.5478652628738 | | 1987 | 4.8706115048624 | | 1986 | 5.4425115593707 | | 1985 | 5.6566129154735 | | 1984 | 4.698815404537 | | 1983 | 4.0714685678683 | | 1982 | 4.6511901748023 | | 1981 | 4.741355162516 | | 1980 | 4.6368613562022 | | 1979 | 4.6520155185466 | | 1978 | 5.195237290077 | | 1977 | 5.6771616943721 | | 1976 | 5.0443610476006 | | 1975 | 4.649256993007 | | 1974 | 4.2941722505207 | | 1973 | 4.7046309932908 | | 1972 | 5.0858079444659 | | 1971 | 5.6417031229696 | | 1970 | 5.3990810074881 |
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