| Year | Value | | 2021 | 51.103490353664 | | 2020 | 51.103490353664 | | 2019 | 51.103490353664 | | 2018 | 50.279445249431 | | 2017 | 50.279445249431 | | 2016 | 50.279445249431 | | 2015 | 50.279445249431 | | 2014 | 48.68557234527 | | 2013 | 48.68557234527 | | 2012 | 48.68557234527 | | 2011 | 49.617056510039 | | 2010 | 49.617056510039 | | 2009 | 49.617056510039 | | 2008 | 49.617056510039 | | 2007 | 50.031049472159 | | 2006 | 50.031049472159 | | 2005 | 50.031049472159 | | 2004 | 50.031049472159 | | 2003 | 50.403643138067 | | 2002 | 52.059614986545 | | 2001 | 52.059614986545 | | 2000 | 52.059614986545 | | 1999 | 51.066031877458 | | 1998 | 51.956116746015 | | 1997 | 51.749120264956 | | 1996 | 53.301593872904 | | 1995 | 53.32229352101 | | 1994 | 52.701304077831 | | 1993 | 52.701304077831 | | 1992 | 52.701304077831 | | 1991 | 52.701304077831 | | 1990 | 52.701304077831 | | 1989 | 54.667770647899 | | 1988 | 54.584972055475 | | 1987 | 54.584972055475 | | 1986 | 54.584972055475 | | 1985 | 54.440074518733 | | 1984 | 54.440074518733 | | 1983 | 54.440074518733 | | 1982 | 54.440074518733 | | 1981 | 54.336576278203 | | 1980 | 54.233078037673 | | 1979 | 53.612088594494 | | 1978 | 53.094597391844 | | 1977 | 52.577106189195 | | 1976 | 51.956116746015 | | 1975 | 50.817636100186 | | 1974 | 50.196646657007 | | 1973 | 49.782653694887 | | 1972 | 49.058166011178 | | 1971 | 48.519975160422 | | 1970 | 48.333678327468 | | 1969 | 48.230180086939 | | 1968 | 47.402194162699 | | 1967 | 47.195197681639 | | 1966 | 46.88470296005 | | 1965 | 46.57420823846 | | 1964 | 46.26371351687 | | 1963 | 45.95321879528 | | 1962 | 45.642724073691 | | 1961 | 45.332229352101 |
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