| Year | Value | | 2023 | 3.0529157896974 | | 2022 | 5.6054063358304 | | 2021 | 2.9049126292848 | | 2020 | 0.038521098005456 | | 2019 | 2.4912352736805 | | 2018 | 2.2658634667874 | | 2017 | 1.5182070073615 | | 2016 | 0.3463769299894 | | 2015 | 1.8614830286337 | | 2014 | 1.5137564305083 | | 2013 | 0.72241478453055 | | 2012 | 1.9733675597625 | | 2011 | 3.1987814166032 | | 2010 | 1.3440273839284 | | 2009 | 2.0627347345268 | | 2008 | 4.4895599537506 | | 2007 | 2.492577949394 | | 2006 | 2.3898262743925 | | 2005 | 1.591606180798 | | 2004 | 0.98191403675766 | | 2003 | 3.0250452625224 | | 2002 | 2.1735779250809 | | 2001 | 2.0449897750512 | | 2000 | 1.6063294174058 | | 1999 | 1.2542482602363 | | 1998 | 1.3366133661337 | | 1997 | 0.54415038337869 | | 1996 | 1.3791374122367 | | 1995 | 2.0669949541004 | | 1994 | 1.3993342374555 | | 1993 | 2.7228976697059 | | 1992 | 5.7381988617345 | | 1991 | 7.1146812020363 | | 1990 | 4.6693191201868 | | 1989 | 5.3876582278481 | | 1988 | 4.4024118278676 | | 1987 | 5.7564640111814 | | 1986 | 5.4337815435623 | | 1985 | 4.6050096339113 | | 1984 | 3.9663461538462 | | 1983 | 3.9999999999999 | | 1982 | 6.012621016675 | | 1981 | 11.114498497389 | | 1980 | 12.097837231068 | | 1979 | 9.0913932679241 | | 1978 | 6.1090703589987 | | 1977 | 3.1898763705574 | | 1976 | 4.2558365758381 | | 1975 | 10.359634997305 | | 1974 | 13.071845838701 |
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