| Year | Value | | 2021 | 49.454828660436 | | 2020 | 48.403426791277 | | 2019 | 47.352024922118 | | 2018 | 46.30062305296 | | 2017 | 45.249221183801 | | 2016 | 44.197819314642 | | 2015 | 43.146417445483 | | 2014 | 42.095015576324 | | 2013 | 41.043613707165 | | 2012 | 39.992211838006 | | 2011 | 38.940809968847 | | 2010 | 36.993769470405 | | 2009 | 36.915887850467 | | 2008 | 36.799065420561 | | 2007 | 36.682242990654 | | 2006 | 36.565420560748 | | 2005 | 37.227414330218 | | 2004 | 38.356697819315 | | 2003 | 38.551401869159 | | 2002 | 38.395638629284 | | 2001 | 37.967289719626 | | 2000 | 37.383177570094 | | 1999 | 37.383177570094 | | 1998 | 36.993769470405 | | 1997 | 36.993769470405 | | 1996 | 36.604361370716 | | 1995 | 36.604361370716 | | 1994 | 36.214953271028 | | 1993 | 36.214953271028 | | 1992 | 36.214953271028 | | 1991 | 36.214953271028 | | 1990 | 36.214953271028 | | 1989 | 36.214953271028 | | 1988 | 36.214953271028 | | 1987 | 36.214953271028 | | 1986 | 36.214953271028 | | 1985 | 36.214953271028 | | 1984 | 36.214953271028 | | 1983 | 36.214953271028 | | 1982 | 36.214953271028 | | 1981 | 36.214953271028 | | 1980 | 36.214953271028 | | 1979 | 36.214953271028 | | 1978 | 36.409657320872 | | 1977 | 36.682242990654 | | 1976 | 36.409657320872 | | 1975 | 36.214953271028 | | 1974 | 36.214953271028 | | 1973 | 36.214953271028 | | 1972 | 34.890965732087 | | 1971 | 36.176012461059 | | 1970 | 36.604361370716 | | 1969 | 38.629283489097 | | 1968 | 31.152647975078 | | 1967 | 30.685358255452 | | 1966 | 29.322429906542 | | 1965 | 29.439252336449 | | 1964 | 28.971962616822 | | 1963 | 28.193146417446 | | 1962 | 27.258566978193 | | 1961 | 26.285046728972 |
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