| Year | Value | | 2021 | 9.0909090909091 | | 2020 | 9.0909090909091 | | 2019 | 9.0909090909091 | | 2018 | 9.0909090909091 | | 2017 | 9.0909090909091 | | 2016 | 9.0909090909091 | | 2015 | 9.0909090909091 | | 2014 | 9.0909090909091 | | 2013 | 9.0909090909091 | | 2012 | 9.0909090909091 | | 2011 | 9.0909090909091 | | 2010 | 9.0909090909091 | | 2009 | 9.0909090909091 | | 2008 | 9.0909090909091 | | 2007 | 9.0909090909091 | | 2006 | 9.0909090909091 | | 2005 | 9.0909090909091 | | 2004 | 9.0909090909091 | | 2003 | 9.0909090909091 | | 2002 | 9.0909090909091 | | 2001 | 9.0909090909091 | | 2000 | 9.0909090909091 | | 1999 | 9.0909090909091 | | 1998 | 9.0909090909091 | | 1997 | 9.0909090909091 | | 1996 | 9.0909090909091 | | 1995 | 9.0909090909091 | | 1994 | 9.0909090909091 | | 1993 | 9.0909090909091 | | 1992 | 9.0909090909091 | | 1991 | 9.0909090909091 | | 1990 | 9.0909090909091 | | 1989 | 9.0909090909091 | | 1988 | 9.0909090909091 | | 1987 | 9.0909090909091 | | 1986 | 9.0909090909091 | | 1985 | 9.0909090909091 | | 1984 | 9.0909090909091 | | 1983 | 11.363636363636 | | 1982 | 11.363636363636 | | 1981 | 11.363636363636 | | 1980 | 11.363636363636 | | 1979 | 11.363636363636 | | 1978 | 11.363636363636 | | 1977 | 11.363636363636 | | 1976 | 11.363636363636 | | 1975 | 11.363636363636 | | 1974 | 11.363636363636 | | 1973 | 11.363636363636 | | 1972 | 11.363636363636 | | 1971 | 13.636363636364 | | 1970 | 13.636363636364 | | 1969 | 13.636363636364 | | 1968 | 13.636363636364 | | 1967 | 13.636363636364 | | 1966 | 13.636363636364 | | 1965 | 13.636363636364 | | 1964 | 15.909090909091 | | 1963 | 15.909090909091 | | 1962 | 15.909090909091 | | 1961 | 18.181818181818 |
|