| Year | Value | | 2021 | 15.243282908106 | | 2020 | 15.243282908106 | | 2019 | 15.038496274554 | | 2018 | 14.833709641002 | | 2017 | 14.628923007451 | | 2016 | 14.424136373899 | | 2015 | 14.219349740348 | | 2014 | 14.014563106796 | | 2013 | 13.809776473245 | | 2012 | 13.604989839693 | | 2011 | 13.400203206141 | | 2010 | 13.19541657259 | | 2009 | 12.990629939038 | | 2008 | 12.785843305487 | | 2007 | 12.156694513434 | | 2006 | 11.527771505983 | | 2005 | 10.898848498532 | | 2004 | 10.269925491081 | | 2003 | 9.6410024836306 | | 2002 | 9.7087378640777 | | 2001 | 9.6297132535561 | | 2000 | 9.7087378640777 | | 1999 | 9.8216301648228 | | 1998 | 9.849853240009 | | 1997 | 10.056446150372 | | 1996 | 10.160307067058 | | 1995 | 10.047414766313 | | 1994 | 10.047414766313 | | 1993 | 10.047414766313 | | 1992 | 9.9345224655679 | | 1991 | 10.16369383608 | | 1990 | 10.160307067058 | | 1989 | 10.160307067058 | | 1988 | 10.160307067058 | | 1987 | 10.160307067058 | | 1986 | 10.160307067058 | | 1985 | 10.160307067058 | | 1984 | 9.0313840596071 | | 1983 | 9.0313840596071 | | 1982 | 9.0313840596071 | | 1981 | 9.0313840596071 | | 1980 | 9.0313840596071 | | 1979 | 9.0313840596071 | | 1978 | 9.0313840596071 | | 1977 | 7.9024610521562 | | 1976 | 7.9024610521562 | | 1975 | 7.9024610521562 | | 1974 | 7.9024610521562 | | 1973 | 7.9024610521562 | | 1972 | 7.9024610521562 | | 1971 | 7.9024610521562 | | 1970 | 7.9024610521562 | | 1969 | 6.7735380447053 | | 1968 | 6.7735380447053 | | 1967 | 6.7735380447053 | | 1966 | 6.7735380447053 | | 1965 | 6.7735380447053 | | 1964 | 6.7735380447053 | | 1963 | 6.8864303454504 | | 1962 | 5.701061187627 | | 1961 | 5.8703996387446 |
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