| Year | Value | | 2021 | 1.0468563399648 | | 2020 | 1.2450116828823 | | 2019 | 1.2870080235404 | | 2018 | 1.3916501116692 | | 2017 | 1.5971029276954 | | 2016 | 1.7982081537348 | | 2015 | 1.9678400910919 | | 2014 | 1.877755265617 | | 2013 | 1.9098306862954 | | 2012 | 1.8149918634645 | | 2011 | 1.8272907293823 | | 2010 | 2.7968904892426 | | 2009 | 2.837368045632 | | 2008 | 3.2806117467972 | | 2007 | 3.0481154917714 | | 2006 | 5.2242980134151 | | 2005 | 7.4602389199124 | | 2004 | 7.6990778201621 | | 2003 | 8.1488211876065 | | 2002 | 9.0853476078066 | | 2001 | 10.624492899532 | | 2000 | 11.611387312686 | | 1999 | 11.971610833734 | | 1998 | 11.71328822406 | | 1997 | 10.431125697172 | | 1996 | 11.592611057448 | | 1995 | 12.650309581533 | | 1994 | 13.073309659746 | | 1993 | 14.354916209402 | | 1992 | 11.879088150973 | | 1991 | 11.197781421099 | | 1990 | 10.689614000375 | | 1989 | 17.655268667132 | | 1988 | 14.745308310992 | | 1987 | 22.710983405437 | | 1986 | 21.463235294118 | | 1985 | 22.582990065423 | | 1984 | 22.357672489632 | | 1983 | 21.243677476941 | | 1982 | 21.120497999111 | | 1981 | 23.227206946454 | | 1980 | 16.259298618491 | | 1979 | 13.737373737374 | | 1978 | 16.393442622951 | | 1977 | 13.986013986014 | | 1976 | 17.821782178218 | | 1975 | 22.784810126582 | | 1974 | 15.254237288136 | | 1973 | 19.565217391304 | | 1972 | 15.555555555556 |
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