| Year | Value | | 2022 | 51.40053352398 | | 2021 | 51.39528585127 | | 2020 | 51.389898057293 | | 2019 | 51.380987630843 | | 2018 | 51.368270682964 | | 2017 | 51.355376989642 | | 2016 | 51.343844923145 | | 2015 | 51.334415756307 | | 2014 | 51.325217107263 | | 2013 | 51.315387808249 | | 2012 | 51.304959619155 | | 2011 | 51.293035675313 | | 2010 | 51.278391936404 | | 2009 | 51.260239836009 | | 2008 | 51.23922691482 | | 2007 | 51.216752196487 | | 2006 | 51.192554853273 | | 2005 | 51.164855596401 | | 2004 | 51.135036720237 | | 2003 | 51.105276081495 | | 2002 | 51.076109322928 | | 2001 | 51.048139800967 | | 2000 | 51.020781614844 | | 1999 | 50.993768617538 | | 1998 | 50.966789300378 | | 1997 | 50.941455582266 | | 1996 | 50.918481819023 | | 1995 | 50.895735428734 | | 1994 | 50.874066407789 | | 1993 | 50.854219123168 | | 1992 | 50.835552304348 | | 1991 | 50.817959892691 | | 1990 | 50.801126989621 | | 1989 | 50.785751527809 | | 1988 | 50.772671921799 | | 1987 | 50.760834205693 | | 1986 | 50.748974788947 | | 1985 | 50.737390724336 | | 1984 | 50.725889617982 | | 1983 | 50.714538935219 | | 1982 | 50.704176392609 | | 1981 | 50.694970203459 | | 1980 | 50.687072921629 | | 1979 | 50.682284954541 | | 1978 | 50.681191378688 | | 1977 | 50.683576713992 | | 1976 | 50.689595978838 | | 1975 | 50.699644627549 | | 1974 | 50.714949896995 | | 1973 | 50.733231114264 | | 1972 | 50.750908303761 | | 1971 | 50.767951543485 | | 1970 | 50.785207095804 | | 1969 | 50.801469599881 | | 1968 | 50.816847295729 | | 1967 | 50.830373127563 | | 1966 | 50.839031005503 | | 1965 | 50.846023533865 | | 1964 | 50.85496705511 | | 1963 | 50.864012759629 | | 1962 | 50.871995388119 | | 1961 | 50.879870977076 | | 1960 | 50.88656002473 |
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