| Year | Value | | 2022 | 51.587405580354 | | 2021 | 51.639947064341 | | 2020 | 51.688739276527 | | 2019 | 51.734558478786 | | 2018 | 51.776733484993 | | 2017 | 51.816639670268 | | 2016 | 51.857482423359 | | 2015 | 51.899178708006 | | 2014 | 51.937931651283 | | 2013 | 51.973427854546 | | 2012 | 52.007258415669 | | 2011 | 52.037977428693 | | 2010 | 52.060476820748 | | 2009 | 52.076943669494 | | 2008 | 52.093250927292 | | 2007 | 52.105369317386 | | 2006 | 52.115100033287 | | 2005 | 52.127802317283 | | 2004 | 52.143801704897 | | 2003 | 52.160292760771 | | 2002 | 52.176018854848 | | 2001 | 52.191361604804 | | 2000 | 52.228011670395 | | 1999 | 52.289834907735 | | 1998 | 52.355916104622 | | 1997 | 52.426569503223 | | 1996 | 52.502886085048 | | 1995 | 52.584102161861 | | 1994 | 52.669972374561 | | 1993 | 52.760772935333 | | 1992 | 52.857098652918 | | 1991 | 52.958197141314 | | 1990 | 53.065584951198 | | 1989 | 53.130698555197 | | 1988 | 53.132375869342 | | 1987 | 53.118633419094 | | 1986 | 53.103792426501 | | 1985 | 53.085368022024 | | 1984 | 53.062129848732 | | 1983 | 53.034088986302 | | 1982 | 53.000837025974 | | 1981 | 52.962364704126 | | 1980 | 52.942487538602 | | 1979 | 52.94440780801 | | 1978 | 52.9463962685 | | 1977 | 52.948444802733 | | 1976 | 52.950606074506 | | 1975 | 52.953663352731 | | 1974 | 52.958552496549 | | 1973 | 52.9664194744 | | 1972 | 52.980005362546 | | 1971 | 52.985174448822 | | 1970 | 52.976084536365 | | 1969 | 52.963484636693 | | 1968 | 52.948273858739 | | 1967 | 52.929265618064 | | 1966 | 52.912625633395 | | 1965 | 52.896887648064 | | 1964 | 52.877504873796 | | 1963 | 52.865365847244 | | 1962 | 52.869878543832 | | 1961 | 52.89147760543 | | 1960 | 52.94923718289 |
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