| Year | Value | | 2021 | 6.9695841419263 | | 2020 | 6.8951082417858 | | 2019 | 6.8206191808225 | | 2018 | 6.7461181554748 | | 2017 | 6.6715800405353 | | 2016 | 6.5971615694401 | | 2015 | 6.5227430983448 | | 2014 | 6.4482049834054 | | 2013 | 6.3736668684659 | | 2012 | 6.2992483973707 | | 2011 | 6.2247102824313 | | 2010 | 6.150291811336 | | 2009 | 6.0757536963966 | | 2008 | 6.0012155814571 | | 2007 | 5.9267971103619 | | 2006 | 5.8522589954224 | | 2005 | 5.7827459219396 | | 2004 | 5.7137114238335 | | 2003 | 5.6446769257275 | | 2002 | 5.5756424276215 | | 2001 | 5.5066079295154 | | 2000 | 5.4375734314094 | | 1999 | 5.3685389333033 | | 1998 | 5.2995044351973 | | 1997 | 5.2304699370913 | | 1996 | 5.1614354389852 | | 1995 | 5.0924009408792 | | 1994 | 5.1310459025573 | | 1993 | 5.264209501157 | | 1992 | 5.3974927436008 | | 1991 | 5.5306563422005 | | 1990 | 5.6639395846444 | | 1989 | 5.7972228270883 | | 1988 | 5.930386425688 | | 1987 | 6.0636696681319 | | 1986 | 6.1968332667316 | | 1985 | 6.3300814535291 | | 1984 | 6.1657258672384 | | 1983 | 6.0464409545665 | | 1982 | 5.9271560418945 | | 1981 | 5.8078711292225 | | 1980 | 5.6886111024702 | | 1979 | 5.3600442203648 | | 1978 | 5.0317415118675 | | 1977 | 4.7033191595259 | | 1976 | 4.3750164510286 | | 1975 | 4.046594098687 | | 1974 | 3.9119947739569 | | 1973 | 3.7776347369152 | | 1972 | 3.6433943437176 | | 1971 | 3.5090343066759 | | 1970 | 3.3746742696341 | | 1969 | 3.3333971433836 | | 1968 | 3.2701055497994 | | 1967 | 3.2068139562151 | | 1966 | 3.1435223626309 | | 1965 | 3.0802307690467 | | 1964 | 3.0169391754625 | | 1963 | 2.9536475818783 | | 1962 | 2.8902363444498 | | 1961 | 2.8269447508656 |
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