| Year | Value | | 2014 | 34.305523751202 | | 2013 | 32.475554041478 | | 2012 | 31.496522282075 | | 2011 | 29.382041992504 | | 2010 | 28.34760874602 | | 2009 | 27.452676301285 | | 2008 | 25.793979456805 | | 2007 | 24.969607228614 | | 2006 | 23.469241708486 | | 2005 | 22.052652629407 | | 2004 | 21.380638358336 | | 2003 | 19.663345995446 | | 2002 | 20.35277550025 | | 2001 | 20.080460840313 | | 2000 | 20.445539766836 | | 1999 | 20.062413222318 | | 1998 | 17.230376255376 | | 1997 | 14.934121714559 | | 1996 | 14.25698889967 | | 1995 | 13.055077485056 | | 1994 | 12.197368852825 | | 1993 | 11.984993226903 | | 1992 | 11.045520694375 | | 1991 | 8.7974428320261 | | 1990 | 8.2598877930643 | | 1989 | 7.6152729128721 | | 1988 | 6.1842350080257 | | 1987 | 5.94714357135 | | 1986 | 5.3009162048763 | | 1985 | 6.3156654345894 | | 1984 | 4.0005581200894 | | 1983 | 4.8156744118125 | | 1982 | 6.552875035962 | | 1981 | 7.8095656622036 | | 1980 | 9.4632316693531 | | 1979 | 9.9026507276974 | | 1978 | 7.4840550265604 | | 1977 | 8.7771711186138 | | 1976 | 8.5735843142006 | | 1975 | 7.2955521945401 | | 1974 | 9.0818149905849 | | 1973 | 9.8315810343553 | | 1972 | 9.2320996712393 | | 1971 | 9.7620316508836 |
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