| Year | Value | | 2021 | 6.312 | | 2020 | 6.417 | | 2019 | 6.531 | | 2018 | 6.632 | | 2017 | 6.738 | | 2016 | 6.886 | | 2015 | 6.978 | | 2014 | 7.057 | | 2013 | 7.128 | | 2012 | 7.198 | | 2011 | 7.257 | | 2010 | 7.299 | | 2009 | 7.34 | | 2008 | 7.387 | | 2007 | 7.42 | | 2006 | 7.458 | | 2005 | 7.484 | | 2004 | 7.522 | | 2003 | 7.547 | | 2002 | 7.578 | | 2001 | 7.582 | | 2000 | 7.61 | | 1999 | 7.632 | | 1998 | 7.647 | | 1997 | 7.662 | | 1996 | 7.621 | | 1995 | 7.581 | | 1994 | 7.544 | | 1993 | 7.526 | | 1992 | 7.5 | | 1991 | 7.47 | | 1990 | 7.44 | | 1989 | 7.424 | | 1988 | 7.398 | | 1987 | 7.373 | | 1986 | 7.349 | | 1985 | 7.327 | | 1984 | 7.3 | | 1983 | 7.277 | | 1982 | 7.252 | | 1981 | 7.227 | | 1980 | 7.178 | | 1979 | 7.161 | | 1978 | 7.13 | | 1977 | 7.004 | | 1976 | 7.016 | | 1975 | 7.033 | | 1974 | 7.057 | | 1973 | 7.086 | | 1972 | 7.118 | | 1971 | 7.151 | | 1970 | 7.182 | | 1969 | 7.209 | | 1968 | 7.231 | | 1967 | 7.248 | | 1966 | 7.257 | | 1965 | 7.261 | | 1964 | 7.261 | | 1963 | 7.258 | | 1962 | 7.255 | | 1961 | 7.252 | | 1960 | 7.25 |
|