| Year | Value | | 2021 | 3.35179375 | | 2020 | 3.074423046875 | | 2019 | 2.7970515625 | | 2018 | 2.97996171875 | | 2017 | 2.93501953125 | | 2016 | 2.967321875 | | 2015 | 3.25580703125 | | 2014 | 3.4528640625 | | 2013 | 3.64993203125 | | 2012 | 3.847 | | 2011 | 3.82046875 | | 2010 | 3.793984375 | | 2009 | 3.7675 | | 2008 | 3.741015625 | | 2007 | 3.71453125 | | 2006 | 3.688046875 | | 2005 | 3.6615625 | | 2004 | 3.635078125 | | 2003 | 3.60859375 | | 2002 | 3.582109375 | | 2001 | 3.555625 | | 2000 | 3.529140625 | | 1999 | 3.50265625 | | 1998 | 3.476171875 | | 1997 | 3.4496875 | | 1996 | 3.423203125 | | 1995 | 3.39671875 | | 1994 | 3.370234375 | | 1993 | 3.328654140625 | | 1992 | 3.287073828125 | | 1991 | 3.24549359375 | | 1990 | 3.203913359375 | | 1989 | 3.162333125 | | 1988 | 3.1207528125 | | 1987 | 3.079172578125 | | 1986 | 3.03759234375 | | 1985 | 2.996012109375 | | 1984 | 2.954431796875 | | 1983 | 2.9128515625 | | 1982 | 2.871271328125 | | 1981 | 2.829691015625 | | 1980 | 2.78811078125 | | 1979 | 2.746530546875 | | 1978 | 2.7049503125 | | 1977 | 2.66337 | | 1976 | 2.621789765625 | | 1975 | 2.58020953125 | | 1974 | 2.538629296875 | | 1973 | 2.497048984375 | | 1972 | 2.45546875 | | 1971 | 1.9984375 | | 1970 | 1.9984375 | | 1969 | 1.92265625 | | 1968 | 1.9140625 | | 1967 | 1.8953125 | | 1966 | 1.88828125 | | 1965 | 1.96171875 | | 1964 | 1.88984375 | | 1963 | 1.71875 | | 1962 | 1.50703125 | | 1961 | 1.403125 |
|