| Year | Value | | 2022 | 8.8336988674945 | | 2021 | 2.5545069964204 | | 2020 | 0.47649885272507 | | 2019 | 1.6305226075434 | | 2018 | 1.7386086198818 | | 2017 | 1.4291074331931 | | 2016 | 0.18333486112386 | | 2015 | 0.061644680064167 | | 2014 | 0.19934382657075 | | 2013 | 1.2199934227431 | | 2012 | 2.6628416550802 | | 2011 | 3.2894493956421 | | 2010 | 1.5311227042093 | | 2009 | 0.83926224679389 | | 2008 | 4.1649719352476 | | 2007 | 2.5106656524034 | | 2006 | 2.6663149457707 | | 2005 | 2.4876966516409 | | 2004 | 2.2862170680168 | | 2003 | 2.0919983899765 | | 2002 | 2.424436612197 | | 2001 | 3.3739683221387 | | 2000 | 3.1507670702259 | | 1999 | 2.1571791805506 | | 1998 | 2.4155178723923 | | 1997 | 3.1120779195327 | | 1996 | 3.5588455933856 | | 1995 | 4.4266455028665 | | 1994 | 4.7184137396733 | | 1993 | 4.8541666666669 | | 1992 | 6.2173753964715 | | 1991 | 5.4847439159201 | | 1990 | 6.1495653117985 | | 1989 | 6.2598313795698 | | 1988 | 4.5351330101572 | | 1987 | 4.0199934568316 | | 1986 | 3.6781841186345 | | 1985 | 5.3954973146638 | | 1984 | 7.6738026224732 | | 1983 | 8.6692715980852 | | 1982 | 9.5849276467744 | | 1981 | 11.767387434418 | | 1980 | 12.911442326638 | | 1979 | 8.2216869300552 | | 1978 | 7.7552459627847 | | 1977 | 9.7674647360704 | | 1976 | 9.3470527960525 | | 1975 | 10.467573464372 | | 1974 | 13.16360261208 | | 1973 | 7.7490861291921 | | 1972 | 6.0073896986249 | | 1971 | 5.2409744827309 | | 1970 | 4.5060889298604 | | 1969 | 2.6746236597134 | | 1968 | 3.2428288767945 | | 1967 | 3.3193236113495 | | 1966 | 3.7033571866951 | | 1965 | 3.9866196012713 | | 1964 | 3.4162803965556 | | 1963 | 2.9216411190019 | | 1962 | 3.5534986597407 | | 1961 | 2.0771383313786 | | 1960 | 1.7411808474692 |
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