| Year | Value | | 2022 | 6.3781469513337 | | 2021 | 2.4107031217436 | | 2020 | 2.1164901463267 | | 2019 | 2.2060730578152 | | 2018 | 2.3241448536789 | | 2017 | 0.90845825587397 | | 2016 | 1.6503899443164 | | 2015 | 1.3748854262144 | | 2014 | 0.9121395454501 | | 2013 | 2.91354193604 | | 2012 | 4.1776358710252 | | 2011 | 4.4408340102897 | | 2010 | 3.3434265756178 | | 2009 | 3.0436184794 | | 2008 | 8.0332732765031 | | 2007 | 4.4109970419349 | | 2006 | 4.2519532197796 | | 2005 | 3.0944901145618 | | 2004 | 2.429766079618 | | 2003 | 2.1549280990868 | | 2002 | 1.9229211590796 | | 2001 | 3.811317483915 | | 2000 | 1.7045041557898 | | 1999 | 3.8232175109556 | | 1998 | 4.3942350998348 | | 1997 | 3.9643780522841 | | 1996 | 5.1857603781201 | | 1995 | 6.9905920974265 | | 1994 | 3.6113580014784 | | 1993 | 4.9488892126269 | | 1992 | 4.5963186428918 | | 1991 | 6.323448045518 | | 1990 | 7.1588809523038 | | 1989 | 5.0740544301489 | | 1988 | 4.037037037037 | | 1987 | 4.3528994898092 | | 1986 | 3.1403239621696 | | 1985 | 4.4209723962191 | | 1984 | 6.0871667357372 | | 1983 | 6.5331087489336 | | 1982 | 7.1299067581671 | | 1981 | 12.30778511939 | | 1980 | 13.858181456248 | | 1979 | 8.2670469761757 | | 1978 | 9.6535231974725 | | 1977 | 10.15994044596 | | 1976 | 10.902496266269 | | 1975 | 17.742787699652 | | 1974 | 20.488615521753 | | 1973 | 10.735033453362 | | 1972 | 5.9533223713126 |
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