| Year | Value | | 2021 | 1.9047619047619 | | 2020 | 1.9047619047619 | | 2019 | 2.8571428571429 | | 2018 | 2.8571428571429 | | 2017 | 2.9523809523809 | | 2016 | 2.8571428571429 | | 2015 | 2.9523809523809 | | 2014 | 2.9523809523809 | | 2013 | 2.9523809523809 | | 2012 | 2.9523809523809 | | 2011 | 3.3333333333333 | | 2010 | 3.3333333333333 | | 2009 | 3.8095238095238 | | 2008 | 3.8095238095238 | | 2007 | 3.8095238095238 | | 2006 | 3.8095238095238 | | 2005 | 4.7619047619048 | | 2004 | 4.7619047619048 | | 2003 | 4.7619047619048 | | 2002 | 4.7619047619048 | | 2001 | 4.7619047619048 | | 2000 | 4.7619047619048 | | 1999 | 4.7619047619048 | | 1998 | 4.7619047619048 | | 1997 | 4.7619047619048 | | 1996 | 5.7142857142857 | | 1995 | 5.7142857142857 | | 1994 | 6.0606060606061 | | 1993 | 6.0606060606061 | | 1992 | 6.0606060606061 | | 1991 | 6.0606060606061 | | 1990 | 6.0606060606061 | | 1989 | 6.0606060606061 | | 1988 | 7.0707070707071 | | 1987 | 7.0707070707071 | | 1986 | 7.0707070707071 | | 1985 | 7.0707070707071 | | 1984 | 7.0707070707071 | | 1983 | 7.0707070707071 | | 1982 | 7.0707070707071 | | 1981 | 7 | | 1980 | 8 | | 1979 | 8 | | 1978 | 8 | | 1977 | 8 | | 1976 | 8.9108910891089 | | 1975 | 8.9108910891089 | | 1974 | 10.78431372549 | | 1973 | 10.78431372549 | | 1972 | 10.78431372549 | | 1971 | 10.78431372549 | | 1970 | 11.764705882353 | | 1969 | 11.764705882353 | | 1968 | 11.764705882353 | | 1967 | 11.764705882353 | | 1966 | 11.764705882353 | | 1965 | 11.764705882353 | | 1964 | 11.764705882353 | | 1963 | 12.745098039216 | | 1962 | 12.745098039216 | | 1961 | 12.745098039216 |
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