| Year | Value | | 2022 | 52.296664033219 | | 2021 | 52.305396680054 | | 2020 | 52.285625958883 | | 2019 | 52.273462197035 | | 2018 | 52.268873686761 | | 2017 | 52.264008922216 | | 2016 | 52.257878374974 | | 2015 | 52.243727365116 | | 2014 | 52.211847334405 | | 2013 | 52.168735071116 | | 2012 | 52.123608879754 | | 2011 | 52.076176064274 | | 2010 | 51.971244812233 | | 2009 | 51.748277287982 | | 2008 | 51.451801648561 | | 2007 | 51.131912980973 | | 2006 | 50.786533983657 | | 2005 | 50.544890171492 | | 2004 | 50.56134072655 | | 2003 | 50.712412629812 | | 2002 | 50.861713553505 | | 2001 | 51.010800621255 | | 2000 | 51.096728292516 | | 1999 | 51.057776401803 | | 1998 | 50.952915363631 | | 1997 | 50.842341308641 | | 1996 | 50.725717523165 | | 1995 | 50.611745617439 | | 1994 | 50.509056994782 | | 1993 | 50.410136854251 | | 1992 | 50.305724714248 | | 1991 | 50.194160134845 | | 1990 | 50.131245059491 | | 1989 | 50.186322878861 | | 1988 | 50.308800126644 | | 1987 | 50.432863829083 | | 1986 | 50.560328352596 | | 1985 | 50.680939719151 | | 1984 | 50.773214142823 | | 1983 | 50.847189985235 | | 1982 | 50.918447889873 | | 1981 | 50.987224489563 | | 1980 | 51.036039469678 | | 1979 | 51.093765702991 | | 1978 | 51.168039957801 | | 1977 | 51.226525478521 | | 1976 | 51.271830856399 | | 1975 | 51.309136925093 | | 1974 | 51.349866436849 | | 1973 | 51.391810319582 | | 1972 | 51.435025439055 | | 1971 | 51.475642921895 | | 1970 | 51.513048353624 | | 1969 | 51.469484973693 | | 1968 | 51.366872358937 | | 1967 | 51.288043837741 | | 1966 | 51.184416570609 | | 1965 | 51.089967973575 | | 1964 | 50.9954870225 | | 1963 | 50.860928354693 | | 1962 | 50.727417560999 | | 1961 | 50.611957776211 | | 1960 | 50.345485763039 |
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