| Year | Value | | 2021 | 6.4403829416884 | | 2020 | 6.4403829416884 | | 2019 | 6.4403829416884 | | 2018 | 6.4403829416884 | | 2017 | 6.266318537859 | | 2016 | 6.1792863359443 | | 2015 | 6.1792863359443 | | 2014 | 6.1792863359443 | | 2013 | 6.046511627907 | | 2012 | 5.9862187769165 | | 2011 | 5.9259259259259 | | 2010 | 5.8656330749354 | | 2009 | 5.8053402239449 | | 2008 | 5.7450473729543 | | 2007 | 5.6847545219638 | | 2006 | 5.6244616709733 | | 2005 | 5.5641688199828 | | 2004 | 5.6158484065461 | | 2003 | 5.7019810508183 | | 2002 | 5.6589147286822 | | 2001 | 5.7105943152455 | | 2000 | 5.6847545219638 | | 1999 | 5.6847545219638 | | 1998 | 5.6847545219638 | | 1997 | 5.6847545219638 | | 1996 | 5.6847545219638 | | 1995 | 5.5986218776916 | | 1994 | 5.5986218776916 | | 1993 | 5.5986218776916 | | 1992 | 5.5124892334195 | | 1991 | 5.2540913006029 | | 1990 | 5.2540913006029 | | 1989 | 5.1679586563308 | | 1988 | 5.2540913006029 | | 1987 | 5.2540913006029 | | 1986 | 5.1679586563308 | | 1985 | 5.0818260120586 | | 1984 | 5.0818260120586 | | 1983 | 4.9956933677864 | | 1982 | 4.9095607235142 | | 1981 | 4.823428079242 | | 1980 | 4.7372954349698 | | 1979 | 4.5650301464255 | | 1978 | 4.4788975021533 | | 1977 | 4.4788975021533 | | 1976 | 4.4788975021533 | | 1975 | 4.4788975021533 | | 1974 | 4.4788975021533 | | 1973 | 4.3927648578811 | | 1972 | 4.3927648578811 | | 1971 | 4.3927648578811 | | 1970 | 4.3927648578811 | | 1969 | 4.3927648578811 | | 1968 | 4.3927648578811 | | 1967 | 4.3927648578811 | | 1966 | 4.3927648578811 | | 1965 | 4.3927648578811 | | 1964 | 4.3927648578811 | | 1963 | 4.3927648578811 | | 1962 | 4.3927648578811 | | 1961 | 4.3927648578811 |
|