| Year | Value | | 2021 | 6.8358206508822 | | 2020 | 6.6809267409174 | | 2019 | 6.5260328309526 | | 2018 | 6.3711389209877 | | 2017 | 6.2162450110229 | | 2016 | 6.061351101058 | | 2015 | 5.905637646596 | | 2014 | 5.7507437366312 | | 2013 | 5.5958498266663 | | 2012 | 5.4409559167015 | | 2011 | 5.2860620067367 | | 2010 | 5.1311680967718 | | 2009 | 5.1265786475877 | | 2008 | 4.9426728624231 | | 2007 | 4.7599144395545 | | 2006 | 4.6525541104254 | | 2005 | 4.591907817635 | | 2004 | 4.0174071251199 | | 2003 | 4.8697334021751 | | 2002 | 4.0157680361255 | | 2001 | 3.7125365721732 | | 2000 | 3.7125365721732 | | 1999 | 3.7125365721732 | | 1998 | 3.7125365721732 | | 1997 | 3.7125365721732 | | 1996 | 3.7125365721732 | | 1995 | 2.7085945631418 | | 1994 | 2.5897606110524 | | 1993 | 2.5078061613355 | | 1992 | 1.7726747473754 | | 1991 | 1.6907202976586 | | 1990 | 1.6825248526869 | | 1989 | 1.6825248526869 | | 1988 | 1.6825248526869 | | 1987 | 1.668592596235 | | 1986 | 1.668592596235 | | 1985 | 1.6661339627435 | | 1984 | 1.6497430728001 | | 1983 | 1.6497430728001 | | 1982 | 1.6497430728001 | | 1981 | 1.6497430728001 | | 1980 | 1.6472844393086 | | 1979 | 1.6472844393086 | | 1978 | 1.6513821617945 | | 1977 | 1.6513821617945 | | 1976 | 1.6530212507888 | | 1975 | 1.4899318958523 | | 1974 | 1.4489546709939 | | 1973 | 1.4079774461354 | | 1972 | 1.4079774461354 | | 1971 | 1.4079774461354 | | 1970 | 1.4079774461354 | | 1969 | 1.367000221277 | | 1968 | 1.367000221277 | | 1967 | 1.367000221277 | | 1966 | 1.362082954294 | | 1965 | 1.3588047763053 | | 1964 | 1.3547070538195 | | 1963 | 1.3506093313336 | | 1962 | 1.3465116088478 | | 1961 | 1.342413886362 |
|