| Year | Value | | 2022 | 57.621293411871 | | 2021 | 57.763608501002 | | 2020 | 58.023696182029 | | 2019 | 58.156916493042 | | 2018 | 58.071328360081 | | 2017 | 57.990086427323 | | 2016 | 57.730264329322 | | 2015 | 57.325786672353 | | 2014 | 57.163650107435 | | 2013 | 57.203316585615 | | 2012 | 57.247218284951 | | 2011 | 57.300054774403 | | 2010 | 57.341046892912 | | 2009 | 57.310175325291 | | 2008 | 57.221937553524 | | 2007 | 57.133830109832 | | 2006 | 57.050621147254 | | 2005 | 56.973697952147 | | 2004 | 57.010457527168 | | 2003 | 57.110774391969 | | 2002 | 57.14470604902 | | 2001 | 57.157808485212 | | 2000 | 57.163521892907 | | 1999 | 57.171929483866 | | 1998 | 57.18037003641 | | 1997 | 57.18288404317 | | 1996 | 57.191127588062 | | 1995 | 57.209395994686 | | 1994 | 57.191131000317 | | 1993 | 57.154071050838 | | 1992 | 57.118876708128 | | 1991 | 57.048111848272 | | 1990 | 56.947153794155 | | 1989 | 56.825617181326 | | 1988 | 56.694097989529 | | 1987 | 56.550122294159 | | 1986 | 56.390730522513 | | 1985 | 56.201515176787 | | 1984 | 55.976092009 | | 1983 | 55.714224903414 | | 1982 | 55.424083982775 | | 1981 | 55.113398676988 | | 1980 | 54.763781114483 | | 1979 | 54.374880319174 | | 1978 | 53.952494114367 | | 1977 | 53.486109256662 | | 1976 | 52.995893050062 | | 1975 | 52.470730140915 | | 1974 | 52.08857987152 | | 1973 | 51.893998920544 | | 1972 | 51.702462427092 | | 1971 | 51.533566434222 | | 1970 | 51.398654251071 | | 1969 | 51.290188632954 | | 1968 | 51.197163321011 | | 1967 | 51.114136969418 | | 1966 | 51.04100198773 | | 1965 | 50.972961616993 | | 1964 | 50.907170399968 | | 1963 | 50.84624447303 | | 1962 | 50.790756386206 | | 1961 | 50.744952216046 | | 1960 | 50.717634086917 |
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