| Year | Value | | 2022 | 2.5597891192934 | | 2021 | 1.2195119391684 | | 2020 | 0.74073259704485 | | 2019 | 0.59814976473575 | | 2018 | 0.80410231327694 | | 2017 | 0.90845825587397 | | 2016 | 1.6503899443164 | | 2015 | 0.51544803143543 | | 2014 | 0.98360305555517 | | 2013 | 0.043999786482617 | | 2012 | 2.4107770402187 | | 2011 | 3.0334728033466 | | 2010 | 3.4365090594342 | | 2009 | 0.30865477958564 | | 2008 | 8.0332732765031 | | 2007 | 3.8588292246965 | | 2006 | 4.2519532197796 | | 2005 | 3.475806937554 | | 2004 | 2.3129167822611 | | 2003 | 2.1549280990868 | | 2002 | 1.0672154799288 | | 2001 | 3.1409930520254 | | 2000 | 2.1818181818182 | | 1999 | 0.57742763867712 | | 1998 | 1.377693303895 | | 1997 | 1.2445278298934 | | 1996 | 2.0323517212778 | | 1995 | 1.8688247529898 | | 1994 | 3.7706576728497 | | 1993 | 2.8085991678224 | | 1992 | 3.7783375314862 | | 1991 | 2.6446036788068 | | 1990 | 2.7242373653053 | | 1989 | 5.592948717949 | | 1988 | 4.0000000000001 | | 1987 | 0.87233338845751 | | 1986 | 0.55707990355053 | | 1985 | 2.5013849235098 | | 1984 | 5.6596127870328 | | 1983 | 6.0953472819335 | | 1982 | 7.8127414121649 | | 1981 | 18.787470940903 | | 1980 | 21.821433894768 | | 1979 | 20.882882882883 | | 1978 | 18.135376756066 | | 1977 | 18.456883509834 |
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