| Year | Value | | 2022 | 51.306368226165 | | 2021 | 51.281099737361 | | 2020 | 51.257137770091 | | 2019 | 51.239696952392 | | 2018 | 51.23326286117 | | 2017 | 51.231899411484 | | 2016 | 51.230608403446 | | 2015 | 51.236501642044 | | 2014 | 51.240749557776 | | 2013 | 51.244585500973 | | 2012 | 51.265063952193 | | 2011 | 51.284133285216 | | 2010 | 51.295947667674 | | 2009 | 51.324815980177 | | 2008 | 51.361751859958 | | 2007 | 51.404047645045 | | 2006 | 51.452782015509 | | 2005 | 51.500375271578 | | 2004 | 51.546530286383 | | 2003 | 51.558548970429 | | 2002 | 51.526305558234 | | 2001 | 51.484444725265 | | 2000 | 51.445547618596 | | 1999 | 51.404477981705 | | 1998 | 51.347130396922 | | 1997 | 51.284629115524 | | 1996 | 51.234589354886 | | 1995 | 51.185952099939 | | 1994 | 51.146175400337 | | 1993 | 51.100779281841 | | 1992 | 51.037097814917 | | 1991 | 50.975271780003 | | 1990 | 50.926071942353 | | 1989 | 50.898423591105 | | 1988 | 50.885038590711 | | 1987 | 50.885298398359 | | 1986 | 50.899315207141 | | 1985 | 50.920575169279 | | 1984 | 50.943220658634 | | 1983 | 50.970416965612 | | 1982 | 51.009769367872 | | 1981 | 51.057802047598 | | 1980 | 51.102788958196 | | 1979 | 51.148821350266 | | 1978 | 51.202700338928 | | 1977 | 51.263024889483 | | 1976 | 51.330799151656 | | 1975 | 51.407079471963 | | 1974 | 51.489259401187 | | 1973 | 51.57552333178 | | 1972 | 51.665819884899 | | 1971 | 51.747011126341 | | 1970 | 51.79380992253 | | 1969 | 51.816715880296 | | 1968 | 51.843774400634 | | 1967 | 51.87744289149 | | 1966 | 51.917193134144 | | 1965 | 51.965269442038 | | 1964 | 52.021260983803 | | 1963 | 52.084253011288 | | 1962 | 52.156970972645 | | 1961 | 52.231130471428 | | 1960 | 52.276945582459 |
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