| Year | Value | | 2014 | 16.677064713836 | | 2013 | 14.594761292296 | | 2012 | 15.76566187301 | | 2011 | 13.505459456125 | | 2010 | 13.067170169365 | | 2009 | 11.437417938938 | | 2008 | 10.069373296471 | | 2007 | 8.8626179637391 | | 2006 | 8.9369786363637 | | 2005 | 10.727110852182 | | 2004 | 9.7706787590361 | | 2003 | 10.485809253889 | | 2002 | 10.036120695204 | | 2001 | 12.378362571214 | | 2000 | 11.955008285318 | | 1999 | 12.659638019278 | | 1998 | 10.128939121552 | | 1997 | 9.8292492522881 | | 1996 | 8.7362769107568 | | 1995 | 8.5407546401792 | | 1994 | 7.9530624025258 | | 1993 | 6.7729481821914 | | 1992 | 6.3378103088257 | | 1991 | 5.9666620654461 | | 1990 | 4.9813202864384 | | 1989 | 3.43140426823 | | 1988 | 4.4340432941053 | | 1987 | 4.2493463289671 | | 1986 | 4.5245130559968 | | 1985 | 3.5668146060635 | | 1984 | 4.7912688763295 | | 1983 | 4.4752772454659 | | 1982 | 3.4656458421456 | | 1981 | 3.016059098863 | | 1980 | 3.484846060208 | | 1979 | 3.126472339609 | | 1978 | 2.145405488946 | | 1977 | 2.213676742951 | | 1976 | 1.9595480725007 | | 1975 | 2.4812169787979 | | 1974 | 2.7715177211909 | | 1973 | 2.9868539693721 | | 1972 | 2.1766500411602 | | 1971 | 1.6561929461091 |
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