| Year | Value | | 2018 | 49.819039141981 | | 2017 | 49.836929929172 | | 2016 | 49.853334337463 | | 2015 | 49.866600602203 | | 2014 | 49.86860282938 | | 2013 | 49.865361316293 | | 2012 | 49.875999770407 | | 2011 | 49.932245248147 | | 2010 | 50.05196450175 | | 2009 | 50.24914366497 | | 2008 | 50.51243017515 | | 2007 | 50.812201226353 | | 2006 | 51.100984502452 | | 2005 | 51.345257583676 | | 2004 | 51.534868238072 | | 2003 | 51.673278879813 | | 2002 | 51.769367091465 | | 2001 | 51.837383894001 | | 2000 | 51.887065659609 | | 1999 | 51.917442015336 | | 1998 | 51.925393866375 | | 1997 | 51.926045397875 | | 1996 | 51.939281930964 | | 1995 | 51.978575570559 | | 1994 | 52.05507838548 | | 1993 | 52.166739059764 | | 1992 | 52.306451204875 | | 1991 | 52.454878129793 | | 1990 | 52.599621840907 | | 1989 | 52.728123678966 | | 1988 | 52.843951078147 | | 1987 | 52.952478225804 | | 1986 | 53.066045929889 | | 1985 | 53.188328606081 | | 1984 | 53.331155680408 | | 1983 | 53.483375856855 | | 1982 | 53.62488275037 | | 1981 | 53.722140239298 | | 1980 | 53.743905188693 | | 1979 | 53.680169020731 | | 1978 | 53.545126040075 | | 1977 | 53.353577112335 | | 1976 | 53.148574692734 | | 1975 | 52.952560686797 | | 1974 | 52.780614888169 | | 1973 | 52.628122832647 | | 1972 | 52.506060528029 | | 1971 | 52.41545804692 | | 1970 | 52.362889146159 | | 1969 | 52.347026694653 | | 1968 | 52.369334306649 | | 1967 | 52.42564297315 | | 1966 | 52.506337600302 | | 1965 | 52.60458031169 | | 1964 | 52.720128818698 | | 1963 | 52.851860593187 | | 1962 | 52.974431453167 | | 1961 | 53.053511331038 | | 1960 | 53.059424096744 |
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