| Year | Value | | 2022 | 4.9947659638104 | | 2021 | 2.30446565742 | | 2020 | 0.70596866133826 | | 2019 | 1.0058200470214 | | 2018 | 2.0876693804698 | | 2017 | 1.5971606033719 | | 2016 | 2.0688403605347 | | 2015 | 1.5579071134626 | | 2014 | 2.3896050363543 | | 2013 | 2.9640049985073 | | 2012 | 3.7312095075689 | | 2011 | 4.5242115050527 | | 2010 | 3.7491572759711 | | 2009 | 2.9208971180536 | | 2008 | 11.270665238085 | | 2007 | 4.1678233011998 | | 2006 | 3.4834069967787 | | 2005 | 3.104966401483 | | 2004 | 3.1222620469935 | | 2003 | 1.9819673462754 | | 2002 | 1.8329938900205 | | 2001 | 1.7466522498543 | | 2000 | 1.8129461388709 | | 1999 | 2.6486225229582 | | 1998 | 3.1253657965586 | | 1997 | 3.5502958579882 | | 1996 | 4.4663061799925 | | 1995 | 6.7410676162314 | | 1994 | 5.1271352169884 | | 1993 | 8.0534659118528 | | 1992 | 7.5920483679552 | | 1991 | 9.0290941811638 | | 1990 | 8.0395686728391 | | 1989 | 5.5414598150856 | | 1988 | 5.999250742737 | | 1987 | 4.0891164464121 | | 1986 | 7.4489713743552 | | 1985 | 4.3893409574154 | | 1984 | 8.116397955171 | | 1983 | 5.4936655143209 | | 1982 | 8.3312557558116 | | 1981 | 11.272159707943 | | 1980 | 10.422699952913 | | 1979 | 7.7392364280668 | | 1978 | 9.7160327495434 | | 1977 | 11.989283322105 | | 1976 | 11.256142506211 | | 1975 | 9.6696954815125 | | 1974 | 14.248935538257 | | 1973 | 8.2666666660001 | | 1972 | 4.0451751325288 | | 1971 | 4.1753252205038 | | 1970 | 3.739278721949 |
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