| Year | Value | | 2022 | 6.6570420275907 | | 2021 | 5.2140494051304 | | 2020 | 1.7774078380616 | | 2019 | 1.9517682105289 | | 2018 | 4.4623110847548 | | 2017 | 4.3213521830319 | | 2016 | 2.7384120873874 | | 2015 | 1.5579071134626 | | 2014 | 2.6662102330264 | | 2013 | 4.0376293364812 | | 2012 | 6.0890964159504 | | 2011 | 4.9714857169238 | | 2010 | 3.9500787592388 | | 2009 | 2.9208971180536 | | 2008 | 12.310736758289 | | 2007 | 4.3258079473263 | | 2006 | 5.0470329614061 | | 2005 | 3.8004680266773 | | 2004 | 3.7970401760691 | | 2003 | 4.3359930878519 | | 2002 | 2.7291356992539 | | 2001 | 2.1265452690465 | | 2000 | 2.6838053534855 | | 1999 | 2.6901248581158 | | 1998 | 3.8725754642749 | | 1997 | 3.6520262072307 | | 1996 | 6.8441608619979 | | 1995 | 11.861094154033 | | 1994 | 11.741349219578 | | 1993 | 12.656466391383 | | 1992 | 12.323299241824 | | 1991 | 14.512331622263 | | 1990 | 12.321799984781 | | 1989 | 10.351789239783 | | 1988 | 6.9050980749377 | | 1987 | 7.4412609128725 | | 1986 | 15.259621308058 | | 1985 | 7.7286389008592 | | 1984 | 9.229518194755 | | 1983 | 6.1676138647939 | | 1982 | 10.393308076463 | | 1981 | 11.846125499795 | | 1980 | 11.584567341648 | | 1979 | 9.9043605123113 | | 1978 | 10.397066566079 | | 1977 | 12.293142157056 | | 1976 | 10.786779196976 | | 1975 | 9.5934364127839 | | 1974 | 12.136626644952 | | 1973 | 7.0718675347066 | | 1972 | 3.7077388435275 | | 1971 | 3.8756212395568 | | 1970 | 4.0592469302555 |
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