| Year | Value | | 2018 | 4.9482796728151 | | 2017 | 6.9441796724661 | | 2016 | 13.012006483186 | | 2015 | 14.441413337162 | | 2014 | 15.099897530901 | | 2013 | 15.152402760808 | | 2012 | 17.318324811388 | | 2011 | 16.787308942449 | | 2010 | 17.108156636472 | | 2009 | 13.3289673625 | | 2008 | 16.206727925749 | | 2007 | 17.331116906939 | | 2006 | 18.176308097788 | | 2005 | 17.719417072691 | | 2004 | 18.829478157648 | | 2003 | 19.052000031453 | | 2002 | 20.656968732949 | | 2001 | 21.400199936918 | | 2000 | 25.0224105831 | | 1999 | 28.009571940938 | | 1998 | 27.97501895138 | | 1997 | 28.623149365343 | | 1996 | 33.113030241841 | | 1995 | 31.597400145635 | | 1994 | 28.388627646434 | | 1993 | 29.303980312433 | | 1992 | 31.176226268473 | | 1991 | 32.628640992904 | | 1990 | 35.793934762219 | | 1989 | 34.854177156358 | | 1988 | 35.423225690384 | | 1987 | 36.191057774002 | | 1986 | 34.265144440763 | | 1985 | 32.939165620204 | | 1984 | 29.65061780997 | | 1983 | 26.369780930924 | | 1982 | 28.202981075197 | | 1981 | 26.628557070168 | | 1980 | 24.800361609161 | | 1979 | 23.354493818911 | | 1978 | 20.961740291957 | | 1977 | 17.835178351784 | | 1976 | 16.903914590747 | | 1975 | 17.373211963589 | | 1974 | 19.125346473668 |
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