| Year | Value | | 2021 | 52.147546282853 | | 2020 | 52.147546282853 | | 2019 | 52.270696932009 | | 2018 | 52.341765332194 | | 2017 | 52.410066897145 | | 2016 | 52.447546100223 | | 2015 | 52.463761763616 | | 2014 | 52.536083001404 | | 2013 | 52.549232317366 | | 2012 | 52.679081812487 | | 2011 | 52.740810545751 | | 2010 | 52.827924763997 | | 2009 | 52.998865871498 | | 2008 | 53.163597579795 | | 2007 | 53.338921792617 | | 2006 | 53.533604720604 | | 2005 | 53.675507755357 | | 2004 | 53.79871321683 | | 2003 | 53.908339067693 | | 2002 | 54.076852695652 | | 2001 | 54.264227026099 | | 2000 | 54.436078142036 | | 1999 | 54.658433869159 | | 1998 | 54.751390699934 | | 1997 | 54.860236026342 | | 1996 | 54.963785187539 | | 1995 | 55.062220809912 | | 1994 | 55.162117443377 | | 1993 | 55.319906641391 | | 1992 | 55.489749180921 | | 1991 | 55.673471325831 | | 1990 | 55.870525196963 | | 1989 | 56.140994875504 | | 1988 | 56.303714985956 | | 1987 | 57.258850987826 | | 1986 | 57.335554070194 | | 1985 | 57.421388471892 | | 1984 | 57.541921887042 | | 1983 | 57.646018927399 | | 1982 | 57.793946300537 | | 1981 | 57.868823119039 | | 1980 | 57.932742354346 | | 1979 | 58.137283907328 | | 1978 | 58.369219418298 | | 1977 | 58.276079961137 | | 1976 | 58.478795250253 | | 1975 | 59.092419909198 | | 1974 | 59.2476523378 | | 1973 | 59.280525087387 | | 1972 | 59.380969600012 | | 1971 | 59.57820609753 | | 1970 | 59.344444322693 | | 1969 | 60.582651223779 | | 1968 | 60.591782543109 | | 1967 | 61.415427546634 | | 1966 | 61.811726805536 | | 1965 | 62.094797704752 | | 1964 | 62.29203420227 | | 1963 | 62.739468849417 | | 1962 | 62.896527541885 | | 1961 | 63.07732766461 |
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