| Year | Value | | 2022 | 50.397643926037 | | 2021 | 50.404663560158 | | 2020 | 50.411584339097 | | 2019 | 50.418459209881 | | 2018 | 50.423096053684 | | 2017 | 50.426534067209 | | 2016 | 50.432011780543 | | 2015 | 50.437611115506 | | 2014 | 50.428038510016 | | 2013 | 50.402394155081 | | 2012 | 50.373855223457 | | 2011 | 50.345142945489 | | 2010 | 50.318688405543 | | 2009 | 50.292764124155 | | 2008 | 50.265659538246 | | 2007 | 50.239251239955 | | 2006 | 50.213021136641 | | 2005 | 50.189100576289 | | 2004 | 50.184504020081 | | 2003 | 50.192799695556 | | 2002 | 50.196134532717 | | 2001 | 50.19657746326 | | 2000 | 50.192870254452 | | 1999 | 50.186627256343 | | 1998 | 50.181153826949 | | 1997 | 50.176389340298 | | 1996 | 50.164931440147 | | 1995 | 50.145856908832 | | 1994 | 50.125105294356 | | 1993 | 50.102960410612 | | 1992 | 50.079498262044 | | 1991 | 50.05367093172 | | 1990 | 50.026596712484 | | 1989 | 49.998903072637 | | 1988 | 49.96840460559 | | 1987 | 49.933694421568 | | 1986 | 49.895436793049 | | 1985 | 49.85498198099 | | 1984 | 49.812799934944 | | 1983 | 49.768402316005 | | 1982 | 49.721906215698 | | 1981 | 49.676223000166 | | 1980 | 49.632375128089 | | 1979 | 49.591427776232 | | 1978 | 49.55642718801 | | 1977 | 49.523377169325 | | 1976 | 49.488202702003 | | 1975 | 49.45016463528 | | 1974 | 49.408188044177 | | 1973 | 49.378866338542 | | 1972 | 49.364951625366 | | 1971 | 49.354223616173 | | 1970 | 49.346854145898 | | 1969 | 49.341471323386 | | 1968 | 49.339218445593 | | 1967 | 49.339406812479 | | 1966 | 49.341831012119 | | 1965 | 49.347918179618 | | 1964 | 49.356507574888 | | 1963 | 49.366866984659 | | 1962 | 49.362558319061 | | 1961 | 49.352483325588 | | 1960 | 49.352989731132 |
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