| Year | Value | | 2021 | 19.96511627907 | | 2020 | 19.96511627907 | | 2019 | 19.96511627907 | | 2018 | 19.797589285714 | | 2017 | 19.483890157807 | | 2016 | 19.170293812292 | | 2015 | 18.858525747508 | | 2014 | 18.54623442691 | | 2013 | 18.236620639535 | | 2012 | 17.926040282392 | | 2011 | 17.614816237542 | | 2010 | 17.307252906977 | | 2009 | 16.996792981728 | | 2008 | 16.686272840532 | | 2007 | 16.374386420266 | | 2006 | 16.069356312292 | | 2005 | 15.75958160299 | | 2004 | 15.449584717608 | | 2003 | 15.135464078073 | | 2002 | 14.821140988372 | | 2001 | 14.50677948505 | | 2000 | 14.193482142857 | | 1999 | 13.87811461794 | | 1998 | 13.566669435216 | | 1997 | 13.242068106312 | | 1996 | 12.927967192691 | | 1995 | 12.609138289036 | | 1994 | 12.293066860465 | | 1993 | 11.980776578073 | | 1992 | 11.668606727575 | | 1991 | 11.358181063123 | | 1990 | 11.044019933555 | | 1989 | 12.082225913621 | | 1988 | 12.082225913621 | | 1987 | 12.113372093023 | | 1986 | 12.051079734219 | | 1985 | 11.957641196013 | | 1984 | 11.936877076412 | | 1983 | 11.926495016611 | | 1982 | 11.874584717608 | | 1981 | 11.833056478405 | | 1980 | 11.833056478405 | | 1979 | 11.833056478405 | | 1978 | 11.833056478405 | | 1977 | 11.833056478405 | | 1976 | 11.781146179402 | | 1975 | 11.781146179402 | | 1974 | 11.781146179402 | | 1973 | 11.781146179402 | | 1972 | 11.781146179402 | | 1971 | 11.781146179402 | | 1970 | 11.781146179402 | | 1969 | 11.801910299003 | | 1968 | 11.801910299003 | | 1967 | 11.822674418605 | | 1966 | 11.822674418605 | | 1965 | 11.853820598007 | | 1964 | 11.853820598007 | | 1963 | 11.884966777409 | | 1962 | 11.884966777409 | | 1961 | 11.90573089701 |
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