| Year | Value | | 2022 | 49.882226532471 | | 2021 | 49.950958240942 | | 2020 | 50.011946914539 | | 2019 | 50.060701844856 | | 2018 | 50.093934229703 | | 2017 | 50.121468176033 | | 2016 | 50.143436641267 | | 2015 | 50.157555244944 | | 2014 | 50.162303552371 | | 2013 | 50.159438090705 | | 2012 | 50.147759848668 | | 2011 | 50.123006742617 | | 2010 | 50.082463816319 | | 2009 | 50.026701554423 | | 2008 | 49.95993819132 | | 2007 | 49.885957520016 | | 2006 | 49.811396266948 | | 2005 | 49.739830127869 | | 2004 | 49.672208976491 | | 2003 | 49.614305376252 | | 2002 | 49.574410662736 | | 2001 | 49.560599084478 | | 2000 | 49.578271167915 | | 1999 | 49.62886424417 | | 1998 | 49.707023908049 | | 1997 | 49.813613283753 | | 1996 | 49.941720969232 | | 1995 | 50.082816642735 | | 1994 | 50.23668798928 | | 1993 | 50.398686828383 | | 1992 | 50.56543510247 | | 1991 | 50.730675595178 | | 1990 | 50.891302328997 | | 1989 | 51.045371646057 | | 1988 | 51.187719103076 | | 1987 | 51.315974791795 | | 1986 | 51.431623558523 | | 1985 | 51.536025348982 | | 1984 | 51.628079851175 | | 1983 | 51.705732451486 | | 1982 | 51.769034312022 | | 1981 | 51.817460898299 | | 1980 | 51.850440449938 | | 1979 | 51.869571854804 | | 1978 | 51.878542230053 | | 1977 | 51.877958551893 | | 1976 | 51.868431138431 | | 1975 | 51.851839273815 | | 1974 | 51.825715580873 | | 1973 | 51.790801274065 | | 1972 | 51.750570530525 | | 1971 | 51.706870665977 | | 1970 | 51.659482322901 | | 1969 | 51.609767063354 | | 1968 | 51.559167848648 | | 1967 | 51.508246644811 | | 1966 | 51.45746174341 | | 1965 | 51.407287033614 | | 1964 | 51.357071502345 | | 1963 | 51.308583055041 | | 1962 | 51.265648998595 | | 1961 | 51.230499033678 | | 1960 | 51.202877063843 |
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