| Year | Value | | 2021 | 8.5302382461166 | | 2020 | 7.7384031655626 | | 2019 | 7.232958825382 | | 2018 | 7.2490916065709 | | 2017 | 7.2641218378257 | | 2016 | 7.3153038271197 | | 2015 | 7.3773509026824 | | 2014 | 7.4413032741102 | | 2013 | 7.521261399256 | | 2012 | 7.6103554642777 | | 2011 | 7.6959750013287 | | 2010 | 7.8106029658402 | | 2009 | 7.8715110406689 | | 2008 | 7.9901793095447 | | 2007 | 8.0070676617637 | | 2006 | 8.0551006030876 | | 2005 | 8.1198683012021 | | 2004 | 8.2055679080967 | | 2003 | 8.2737258661072 | | 2002 | 8.3455401860155 | | 2001 | 8.4217521770458 | | 2000 | 8.4993154530579 | | 1999 | 8.5811948608606 | | 1998 | 8.7245948607327 | | 1997 | 8.7951570374585 | | 1996 | 8.9403354754877 | | 1995 | 9.046763885598 | | 1994 | 9.224430175028 | | 1993 | 9.2543181721156 | | 1992 | 9.2844387824679 | | 1991 | 9.368901273831 | | 1990 | 9.4082370137276 | | 1989 | 9.4565188757357 | | 1988 | 9.731155466021 | | 1987 | 9.7988885102839 | | 1986 | 9.9848983779008 | | 1985 | 10.205798488278 | | 1984 | 10.379210174289 | | 1983 | 10.50801734501 | | 1982 | 10.478796484085 | | 1981 | 10.559890711712 | | 1980 | 10.714499848401 | | 1979 | 10.800084076249 | | 1978 | 10.980112657662 | | 1977 | 11.298354260206 | | 1976 | 11.698472459087 | | 1975 | 11.893298884895 | | 1974 | 11.981319196969 | | 1973 | 12.057125178716 | | 1972 | 12.454003470963 | | 1971 | 13.084760801847 | | 1970 | 12.887503925602 | | 1969 | 13.057589210061 | | 1968 | 13.249462164426 | | 1967 | 13.657489919117 | | 1966 | 13.967790273302 | | 1965 | 14.497361160963 | | 1964 | 14.766746202552 | | 1963 | 14.592589915125 | | 1962 | 14.800008940727 | | 1961 | 16.141008125649 | | 1960 | 19.585625675788 |
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