| Year | Value | | 2014 | 5.9263144174666 | | 2013 | 5.5857684883817 | | 2012 | 4.9462483260298 | | 2011 | 4.9374161143612 | | 2010 | 4.5677425919361 | | 2009 | 4.790249217888 | | 2008 | 4.8437528299722 | | 2007 | 4.6081477015317 | | 2006 | 4.2470623884895 | | 2005 | 4.0935881890865 | | 2004 | 4.4741079995153 | | 2003 | 4.3988297748733 | | 2002 | 4.1862329054959 | | 2001 | 4.1320482220557 | | 2000 | 3.4270853918071 | | 1999 | 3.2256704282493 | | 1998 | 3.3251078327965 | | 1997 | 3.3117232694654 | | 1996 | 3.5135082134614 | | 1995 | 3.3432463542146 | | 1994 | 3.0482712924944 | | 1993 | 2.7366293444389 | | 1992 | 2.2449679995519 | | 1991 | 3.4008915215403 | | 1990 | 3.5503313604813 | | 1989 | 3.4745824715701 | | 1988 | 3.3860652843341 | | 1987 | 3.4555275369988 | | 1986 | 2.919622151323 | | 1985 | 2.6006953376924 | | 1984 | 2.6149125775352 | | 1983 | 2.901180930815 | | 1982 | 2.8367374532086 | | 1981 | 3.1317007617269 | | 1980 | 3.0052313901617 | | 1979 | 2.9534700734674 | | 1978 | 2.8409712326168 | | 1977 | 2.5875897796582 | | 1976 | 2.6571545033848 | | 1975 | 2.6751813567039 | | 1974 | 3.0731109942537 | | 1973 | 3.1663144900558 | | 1972 | 3.0816799277814 | | 1971 | 3.4440015829433 |
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