| Year | Value | | 2022 | 50.988785841813 | | 2021 | 50.993421664296 | | 2020 | 50.990559099272 | | 2019 | 50.981825863249 | | 2018 | 50.967693374705 | | 2017 | 50.93945003206 | | 2016 | 50.90428991347 | | 2015 | 50.870784652315 | | 2014 | 50.832851048103 | | 2013 | 50.774872740669 | | 2012 | 50.699284426783 | | 2011 | 50.634810347407 | | 2010 | 50.588794671702 | | 2009 | 50.548596555383 | | 2008 | 50.538424277243 | | 2007 | 50.588275830883 | | 2006 | 50.650553991032 | | 2005 | 50.710496378033 | | 2004 | 50.799657055062 | | 2003 | 50.885000261661 | | 2002 | 50.966702432912 | | 2001 | 51.01457656204 | | 2000 | 51.017961937772 | | 1999 | 51.022590468132 | | 1998 | 51.02621484667 | | 1997 | 51.030060938202 | | 1996 | 51.031854733791 | | 1995 | 51.032899436093 | | 1994 | 51.035519754894 | | 1993 | 51.040520849204 | | 1992 | 51.045584091587 | | 1991 | 51.048077646258 | | 1990 | 51.033200429037 | | 1989 | 51.005742532857 | | 1988 | 50.984012520125 | | 1987 | 50.967360169459 | | 1986 | 50.955465871562 | | 1985 | 50.945720931006 | | 1984 | 50.938299514022 | | 1983 | 50.935088405062 | | 1982 | 50.935611884414 | | 1981 | 50.940103815685 | | 1980 | 50.94855747408 | | 1979 | 50.960372250209 | | 1978 | 50.977220048597 | | 1977 | 50.997628740296 | | 1976 | 51.018479615299 | | 1975 | 51.041784144045 | | 1974 | 51.096548625658 | | 1973 | 51.150953659517 | | 1972 | 51.175219475958 | | 1971 | 51.195448874427 | | 1970 | 51.217412371466 | | 1969 | 51.237714046906 | | 1968 | 51.253944007556 | | 1967 | 51.272039832482 | | 1966 | 51.290975726034 | | 1965 | 51.313254555966 | | 1964 | 51.33601571619 | | 1963 | 51.358514317034 | | 1962 | 51.383077697445 | | 1961 | 51.40764335918 | | 1960 | 51.438819650093 |
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