| Year | Value | | 2015 | 61.400169368738 | | 2014 | 60.875910279102 | | 2013 | 62.093783982408 | | 2012 | 60.69545859916 | | 2011 | 60.564510222571 | | 2010 | 60.664409708695 | | 2009 | 59.317546459835 | | 2008 | 60.003153519574 | | 2007 | 58.464230046653 | | 2006 | 60.01125617565 | | 2005 | 59.466212575833 | | 2004 | 59.738510821795 | | 2003 | 59.977743006388 | | 2002 | 60.340805304662 | | 2001 | 61.176461938025 | | 2000 | 59.823394858565 | | 1999 | 59.080269275533 | | 1998 | 60.360843053717 | | 1997 | 58.393154702127 | | 1996 | 58.850527862723 | | 1995 | 56.935097128706 | | 1994 | 56.413973225488 | | 1993 | 54.572366308446 | | 1992 | 51.69083384601 | | 1991 | 51.117853655038 | | 1990 | 46.993619579061 | | 1989 | 43.607117207139 | | 1988 | 44.072169004534 | | 1987 | 44.473926304966 | | 1986 | 43.907633617884 | | 1985 | 41.399944214232 | | 1984 | 42.899867578683 | | 1983 | 43.103593944963 | | 1982 | 42.116231903046 | | 1981 | 44.2496863212 | | 1980 | 48.029802871083 | | 1979 | 50.232088047547 | | 1978 | 50.061698208027 | | 1977 | 48.859957879103 | | 1976 | 48.495202386443 | | 1975 | 45.085998694162 | | 1974 | 47.271854438353 | | 1973 | 48.711535832807 | | 1972 | 45.626418813275 | | 1971 | 42.564090085243 | | 1970 | 42.116742085607 | | 1969 | 46.009835136923 | | 1968 | 42.765947430241 | | 1967 | 40.010187956386 | | 1966 | 34.637713611288 | | 1965 | 30.206905422279 | | 1964 | 25.325523274953 | | 1963 | 23.806309340625 | | 1962 | 19.126008315953 | | 1961 | 13.582347551951 | | 1960 | 11.897830899402 |
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