| Year | Value | | 2022 | 49.341944904466 | | 2021 | 49.340252319924 | | 2020 | 49.341138044302 | | 2019 | 49.337938896313 | | 2018 | 49.33227857477 | | 2017 | 49.320575790384 | | 2016 | 49.300426885613 | | 2015 | 49.267438323918 | | 2014 | 49.21988435578 | | 2013 | 49.162293646969 | | 2012 | 49.096725248025 | | 2011 | 49.028443649194 | | 2010 | 48.95926214161 | | 2009 | 48.890057009783 | | 2008 | 48.823277973577 | | 2007 | 48.764734307312 | | 2006 | 48.717215443229 | | 2005 | 48.681292438838 | | 2004 | 48.658969956848 | | 2003 | 48.645666621715 | | 2002 | 48.637409376204 | | 2001 | 48.633744829513 | | 2000 | 48.631793310639 | | 1999 | 48.626496829547 | | 1998 | 48.614880041472 | | 1997 | 48.594853286603 | | 1996 | 48.562293894413 | | 1995 | 48.515856031529 | | 1994 | 48.455059156086 | | 1993 | 48.3815160143 | | 1992 | 48.299081740883 | | 1991 | 48.209257644058 | | 1990 | 48.112330477299 | | 1989 | 48.012965308951 | | 1988 | 47.917049631914 | | 1987 | 47.827437500876 | | 1986 | 47.746249560928 | | 1985 | 47.677611176102 | | 1984 | 47.622715034641 | | 1983 | 47.576353751816 | | 1982 | 47.535454963978 | | 1981 | 47.499816855331 | | 1980 | 47.470219942347 | | 1979 | 47.446159361383 | | 1978 | 47.426271765147 | | 1977 | 47.408794375013 | | 1976 | 47.391600268163 | | 1975 | 47.374765853994 | | 1974 | 47.357704286262 | | 1973 | 47.340787613523 | | 1972 | 47.321879748164 | | 1971 | 47.295541725037 | | 1970 | 47.264502321955 | | 1969 | 47.232214177 | | 1968 | 47.195033271847 | | 1967 | 47.151964093191 | | 1966 | 47.104258442546 | | 1965 | 47.052119791099 | | 1964 | 46.996252945627 | | 1963 | 46.936396529252 | | 1962 | 46.872800415697 | | 1961 | 46.803551518548 | | 1960 | 46.729775985187 |
|