| Year | Value | | 2021 | 5.956 | | 2020 | 6.035 | | 2019 | 6.103 | | 2018 | 6.183 | | 2017 | 6.251 | | 2016 | 6.319 | | 2015 | 6.385 | | 2014 | 6.441 | | 2013 | 6.491 | | 2012 | 6.523 | | 2011 | 6.545 | | 2010 | 6.579 | | 2009 | 6.611 | | 2008 | 6.642 | | 2007 | 6.655 | | 2006 | 6.688 | | 2005 | 6.716 | | 2004 | 6.741 | | 2003 | 6.782 | | 2002 | 6.821 | | 2001 | 6.852 | | 2000 | 6.874 | | 1999 | 6.882 | | 1998 | 6.892 | | 1997 | 6.911 | | 1996 | 6.95 | | 1995 | 6.989 | | 1994 | 7.065 | | 1993 | 7.123 | | 1992 | 7.181 | | 1991 | 7.225 | | 1990 | 7.253 | | 1989 | 7.275 | | 1988 | 7.259 | | 1987 | 7.245 | | 1986 | 7.244 | | 1985 | 7.212 | | 1984 | 7.221 | | 1983 | 7.23 | | 1982 | 7.267 | | 1981 | 7.317 | | 1980 | 7.325 | | 1979 | 7.318 | | 1978 | 7.296 | | 1977 | 7.282 | | 1976 | 7.245 | | 1975 | 7.239 | | 1974 | 7.229 | | 1973 | 7.214 | | 1972 | 7.203 | | 1971 | 7.195 | | 1970 | 7.177 | | 1969 | 7.171 | | 1968 | 7.152 | | 1967 | 7.129 | | 1966 | 7.099 | | 1965 | 7.082 | | 1964 | 7.048 | | 1963 | 7.031 | | 1962 | 7.019 | | 1961 | 7.004 | | 1960 | 7.004 |
|