| Year | Value | | 2021 | 4.4940156927906 | | 2020 | 4.3909111972676 | | 2019 | 4.3777549155359 | | 2018 | 4.2717615618942 | | 2017 | 4.0843224406905 | | 2016 | 4.1892653004708 | | 2015 | 4.1584375519247 | | 2014 | 4.1685985414936 | | 2013 | 4.163205021693 | | 2012 | 4.0778177790086 | | 2011 | 3.9924305363242 | | 2010 | 3.9079663989661 | | 2009 | 3.829040893566 | | 2008 | 3.7501153881658 | | 2007 | 3.6711898827656 | | 2006 | 3.5922643773655 | | 2005 | 3.5133388719653 | | 2004 | 3.3037939628912 | | 2003 | 3.2437921166805 | | 2002 | 3.0462475768485 | | 2001 | 2.9077817779009 | | 2000 | 2.902243145943 | | 1999 | 2.8339333517954 | | 1998 | 2.7453152404689 | | 1997 | 2.6511584971845 | | 1996 | 2.4831533277947 | | 1995 | 2.4000738484261 | | 1994 | 2.1692975168467 | | 1993 | 2.030831717899 | | 1992 | 1.9846764515831 | | 1991 | 1.9477522385304 | | 1990 | 1.9385211852672 | | 1989 | 1.9292901320041 | | 1988 | 1.9200590787409 | | 1987 | 1.9108280254777 | | 1986 | 1.9015969722145 | | 1985 | 1.8923659189513 | | 1984 | 1.932059447983 | | 1983 | 1.8932890242777 | | 1982 | 1.8628265485092 | | 1981 | 1.8277485461091 | | 1980 | 1.7935936490353 | | 1979 | 1.7631311732669 | | 1978 | 1.7326686974984 | | 1977 | 1.7031293270562 | | 1976 | 1.6680513246561 | | 1975 | 1.6172805317087 | | 1974 | 1.5794332133296 | | 1973 | 1.5489707375612 | | 1972 | 1.5083541032032 | | 1971 | 1.4686605741715 | | 1970 | 1.4437367303609 | | 1969 | 1.4095818332872 | | 1968 | 1.3735807255608 | | 1967 | 1.3366565125081 | | 1966 | 1.3052709314133 | | 1965 | 1.2692698236869 | | 1964 | 1.2378842425921 | | 1963 | 1.2037293455183 | | 1962 | 1.1963445029078 | | 1961 | 1.1944982922551 |
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