| Year | Value | | 2021 | 8.4175084175084 | | 2020 | 8.4175084175084 | | 2019 | 8.4175084175084 | | 2018 | 8.4175084175084 | | 2017 | 8.4960718294052 | | 2016 | 8.4175084175084 | | 2015 | 8.3838383838384 | | 2014 | 8.5353535353535 | | 2013 | 8.6195286195286 | | 2012 | 8.5297418630752 | | 2011 | 8.5578002244669 | | 2010 | 8.5297418630752 | | 2009 | 8.5016835016835 | | 2008 | 8.4736251402918 | | 2007 | 8.4736251402918 | | 2006 | 8.4175084175084 | | 2005 | 8.4175084175084 | | 2004 | 8.4175084175084 | | 2003 | 8.4736251402918 | | 2002 | 8.4736251402918 | | 2001 | 8.4736251402918 | | 2000 | 8.3052749719416 | | 1999 | 8.0808080808081 | | 1998 | 8.0246913580247 | | 1997 | 8.0246913580247 | | 1996 | 8.0246913580247 | | 1995 | 7.9685746352413 | | 1994 | 7.9685746352413 | | 1993 | 7.9685746352413 | | 1992 | 7.9124579124579 | | 1991 | 7.9124579124579 | | 1990 | 7.9124579124579 | | 1989 | 7.9124579124579 | | 1988 | 7.9124579124579 | | 1987 | 7.7441077441077 | | 1986 | 7.7441077441077 | | 1985 | 7.6879910213243 | | 1984 | 7.6879910213243 | | 1983 | 7.631874298541 | | 1982 | 7.631874298541 | | 1981 | 7.631874298541 | | 1980 | 7.5757575757576 | | 1979 | 7.5757575757576 | | 1978 | 7.5757575757576 | | 1977 | 7.5757575757576 | | 1976 | 7.5757575757576 | | 1975 | 7.5757575757576 | | 1974 | 7.5757575757576 | | 1973 | 7.5757575757576 | | 1972 | 7.5757575757576 | | 1971 | 7.5757575757576 | | 1970 | 7.5757575757576 | | 1969 | 7.5757575757576 | | 1968 | 7.5757575757576 | | 1967 | 7.5757575757576 | | 1966 | 7.5757575757576 | | 1965 | 7.5757575757576 | | 1964 | 7.5757575757576 | | 1963 | 7.5757575757576 | | 1962 | 7.5757575757576 | | 1961 | 7.5757575757576 |
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