| Year | Value | | 2021 | 4.973 | | 2020 | 5.048 | | 2019 | 5.13 | | 2018 | 5.199 | | 2017 | 5.269 | | 2016 | 5.303 | | 2015 | 5.319 | | 2014 | 5.348 | | 2013 | 5.345 | | 2012 | 5.347 | | 2011 | 5.366 | | 2010 | 5.39 | | 2009 | 5.436 | | 2008 | 5.483 | | 2007 | 5.563 | | 2006 | 5.636 | | 2005 | 5.676 | | 2004 | 5.721 | | 2003 | 5.77 | | 2002 | 5.777 | | 2001 | 5.824 | | 2000 | 5.89 | | 1999 | 5.998 | | 1998 | 6.097 | | 1997 | 6.204 | | 1996 | 6.261 | | 1995 | 6.314 | | 1994 | 6.413 | | 1993 | 6.506 | | 1992 | 6.596 | | 1991 | 6.667 | | 1990 | 6.732 | | 1989 | 6.797 | | 1988 | 6.847 | | 1987 | 6.896 | | 1986 | 6.948 | | 1985 | 6.985 | | 1984 | 7.018 | | 1983 | 7.034 | | 1982 | 7.051 | | 1981 | 7.052 | | 1980 | 6.99 | | 1979 | 6.911 | | 1978 | 6.843 | | 1977 | 6.854 | | 1976 | 6.857 | | 1975 | 6.851 | | 1974 | 6.842 | | 1973 | 6.831 | | 1972 | 6.824 | | 1971 | 6.788 | | 1970 | 6.743 | | 1969 | 6.712 | | 1968 | 6.683 | | 1967 | 6.65 | | 1966 | 6.638 | | 1965 | 6.606 | | 1964 | 6.565 | | 1963 | 6.51 | | 1962 | 6.457 | | 1961 | 6.402 | | 1960 | 6.282 |
|