| Year | Value | | 2019 | 6.0176524772648 | | 2018 | 2.7569712527277 | | 2017 | 6.0777423289658 | | 2016 | 7.175859836402 | | 2015 | 6.0060294299884 | | 2014 | 5.8358222560339 | | 2013 | 5.5295675887451 | | 2012 | 4.5240936974119 | | 2005 | 3.8817783564094 | | 2004 | 3.2724566560911 | | 2003 | 2.2104118642054 | | 2002 | 2.0010471703619 | | 2001 | 2.2571404110197 | | 2000 | 2.9665074581845 | | 1999 | 2.7527484568465 | | 1998 | 3.5193095188399 | | 1997 | 4.415239180748 | | 1996 | 3.9593171544736 | | 1995 | 3.7444872000516 | | 1994 | 4.2517134504619 | | 1993 | 4.7280103529994 | | 1992 | 5.0373965445136 | | 1991 | 5.6102062928528 | | 1990 | 6.1977841457299 | | 1989 | 4.2617873765401 | | 1988 | 4.4935267147344 | | 1987 | 6.3655418074506 | | 1986 | 7.4117367024228 | | 1985 | 8.2551932644781 | | 1984 | 8.5844357440777 | | 1983 | 9.2366981886889 | | 1982 | 9.8310224548756 | | 1981 | 9.9277052238806 | | 1980 | 9.6117489543578 | | 1979 | 10.211416742034 | | 1978 | 10.009433804883 | | 1977 | 10.456479203378 | | 1976 | 8.8051918678106 | | 1975 | 7.9226476302539 | | 1974 | 7.612796448263 | | 1973 | 9.4247975336617 |
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