| Year | Value | | 2015 | 6.8490016570956 | | 2011 | 7.0050147756783 | | 2010 | 8.0547938777725 | | 2009 | 8.2684577574075 | | 2008 | 9.7386718058126 | | 2007 | 8.9503060116842 | | 2006 | 9.7707174042662 | | 2005 | 10.116209694877 | | 2004 | 13.613618085962 | | 2003 | 14.64224199799 | | 2002 | 14.198259911027 | | 1997 | 19.929328621908 | | 1996 | 20.606590114828 | | 1995 | 20.450057022058 | | 1994 | 19.755197551975 | | 1993 | 18.638829750704 | | 1992 | 19.221193495682 | | 1991 | 20.639324487334 | | 1990 | 23.95299313826 | | 1989 | 27.97484984985 | | 1988 | 27.865143115737 | | 1987 | 25.706289978678 | | 1986 | 26.775045157705 | | 1985 | 32.209446861405 | | 1984 | 34.84302624807 | | 1983 | 30.652639087018 | | 1982 | 35.036945812808 | | 1981 | 33.379855780414 | | 1979 | 35.679702048417 | | 1978 | 34.072759538598 | | 1977 | 26.587112171838 | | 1976 | 34.33734939759 | | 1975 | 46.097337006428 |
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