| Year | Value | | 2021 | 8.4272472297356 | | 2020 | 7.7655930201654 | | 2019 | 6.5704562349998 | | 2018 | 6.5273673170812 | | 2017 | 6.455348965752 | | 2016 | 6.4170512837852 | | 2015 | 6.262189154493 | | 2014 | 6.1651493902346 | | 2013 | 6.1370872778659 | | 2012 | 6.1334205979557 | | 2011 | 6.1101496409629 | | 2010 | 6.2475065828444 | | 2009 | 6.042539841552 | | 2008 | 6.0119901077625 | | 2007 | 6.0010431182872 | | 2006 | 5.938318995996 | | 2005 | 5.920430358114 | | 2004 | 5.9796792689933 | | 2003 | 6.0221844597317 | | 2002 | 5.9913190948338 | | 2001 | 6.0436252338793 | | 2000 | 6.0643878900731 | | 1999 | 6.1087571668916 | | 1998 | 6.195657736226 | | 1997 | 6.2198611371621 | | 1996 | 6.2898556121321 | | 1995 | 6.3750443527891 | | 1994 | 6.4264512771287 | | 1993 | 6.5320385033425 | | 1992 | 6.6185432777948 | | 1991 | 6.7154799401002 | | 1990 | 6.8338023692515 | | 1989 | 6.917920286842 | | 1988 | 7.0318209711514 | | 1987 | 7.1830262087426 | | 1986 | 7.3458915512124 | | 1985 | 7.5827443192338 | | 1984 | 7.7189775827003 | | 1983 | 7.8893976886319 | | 1982 | 8.1381850367779 | | 1981 | 8.2692185727948 | | 1980 | 8.4785910824486 | | 1979 | 8.6390880813524 | | 1978 | 8.7724956026288 | | 1977 | 8.9199815117486 | | 1976 | 9.1810979157652 | | 1975 | 9.2589766839177 | | 1974 | 9.5136836947098 | | 1973 | 9.7145554914857 | | 1972 | 9.9877100906741 | | 1971 | 10.174937363415 | | 1970 | 10.650082664061 | | 1969 | 10.697367272227 | | 1968 | 10.91933038793 | | 1967 | 11.168450600353 | | 1966 | 11.439061567058 | | 1965 | 11.723201819051 | | 1964 | 12.006848825881 | | 1963 | 12.336982858243 | | 1962 | 12.60631404102 | | 1961 | 12.91390454027 | | 1960 | 13.279689540927 |
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