| Year | Value | | 2022 | 51.549143769681 | | 2021 | 51.56549707261 | | 2020 | 51.585786999801 | | 2019 | 51.605919830452 | | 2018 | 51.625366715669 | | 2017 | 51.649436035299 | | 2016 | 51.674543597892 | | 2015 | 51.697945643875 | | 2014 | 51.717447080517 | | 2013 | 51.733181906908 | | 2012 | 51.746608632765 | | 2011 | 51.7582058979 | | 2010 | 51.767988973502 | | 2009 | 51.776232926933 | | 2008 | 51.783026180666 | | 2007 | 51.787435811084 | | 2006 | 51.788198593617 | | 2005 | 51.784764284519 | | 2004 | 51.759474995632 | | 2003 | 51.710379004061 | | 2002 | 51.656297511433 | | 2001 | 51.602559150593 | | 2000 | 51.554298856943 | | 1999 | 51.514335477406 | | 1998 | 51.479043474343 | | 1997 | 51.444422291175 | | 1996 | 51.41030913649 | | 1995 | 51.375650656111 | | 1994 | 51.340759793664 | | 1993 | 51.306078322136 | | 1992 | 51.271192352426 | | 1991 | 51.236439416797 | | 1990 | 51.201439070494 | | 1989 | 51.16632648878 | | 1988 | 51.131537221876 | | 1987 | 51.095614045698 | | 1986 | 51.057459132153 | | 1985 | 51.024033320349 | | 1984 | 50.996341732998 | | 1983 | 50.968217192817 | | 1982 | 50.939284676129 | | 1981 | 50.908102152473 | | 1980 | 50.875023275497 | | 1979 | 50.841883084092 | | 1978 | 50.808451589971 | | 1977 | 50.773013827597 | | 1976 | 50.734981290819 | | 1975 | 50.679138577418 | | 1974 | 50.606429519608 | | 1973 | 50.533466110881 | | 1972 | 50.461028128884 | | 1971 | 50.394048530525 | | 1970 | 50.332674495075 | | 1969 | 50.273418081102 | | 1968 | 50.220972350955 | | 1967 | 50.175430761679 | | 1966 | 50.132876251833 | | 1965 | 50.092317380775 | | 1964 | 50.052971373591 | | 1963 | 50.017613849518 | | 1962 | 49.98478268165 | | 1961 | 49.950276301338 | | 1960 | 49.913859948178 |
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