| Year | Value | | 2015 | 65.236456446316 | | 2014 | 64.201667986739 | | 2013 | 61.288297869234 | | 2012 | 64.990391228826 | | 2011 | 70.612662502398 | | 2010 | 70.146616793137 | | 2009 | 65.461598910665 | | 2008 | 67.93482308873 | | 2007 | 69.405801456998 | | 2006 | 67.412798944949 | | 2005 | 67.08240283724 | | 2004 | 68.912678966264 | | 2003 | 66.854350452874 | | 2002 | 64.50883387306 | | 2001 | 63.24148868668 | | 2000 | 65.907058456346 | | 1999 | 68.203564103886 | | 1998 | 65.583784669777 | | 1997 | 63.238207041965 | | 1996 | 59.000470367812 | | 1995 | 54.735055156372 | | 1994 | 51.28091429008 | | 1993 | 48.158445749023 | | 1992 | 44.288174628786 | | 1991 | 41.721151649893 | | 1990 | 43.411613168486 | | 1989 | 43.235198474242 | | 1988 | 38.032126062765 | | 1987 | 31.544832539177 | | 1986 | 29.952745389594 | | 1985 | 28.962026457756 | | 1984 | 29.712648099988 | | 1983 | 28.948075506899 | | 1982 | 28.035201682345 | | 1981 | 35.47217316928 | | 1980 | 38.800926149632 | | 1979 | 45.329737891929 | | 1978 | 38.747913225215 | | 1977 | 37.050644025656 | | 1976 | 35.869489764795 | | 1975 | 34.691681715728 | | 1974 | 39.605829901946 | | 1973 | 40.228017550996 | | 1972 | 38.827344728422 | | 1971 | 38.604359608348 |
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