| Year | Value | | 2021 | 4.463 | | 2020 | 4.543 | | 2019 | 4.623 | | 2018 | 4.694 | | 2017 | 4.774 | | 2016 | 4.818 | | 2015 | 4.832 | | 2014 | 4.855 | | 2013 | 4.914 | | 2012 | 4.989 | | 2011 | 5.058 | | 2010 | 5.16 | | 2009 | 5.227 | | 2008 | 5.278 | | 2007 | 5.331 | | 2006 | 5.354 | | 2005 | 5.407 | | 2004 | 5.45 | | 2003 | 5.463 | | 2002 | 5.458 | | 2001 | 5.443 | | 2000 | 5.53 | | 1999 | 5.629 | | 1998 | 5.668 | | 1997 | 5.723 | | 1996 | 5.828 | | 1995 | 5.955 | | 1994 | 6.082 | | 1993 | 6.194 | | 1992 | 6.29 | | 1991 | 6.347 | | 1990 | 6.392 | | 1989 | 6.437 | | 1988 | 6.5 | | 1987 | 6.563 | | 1986 | 6.588 | | 1985 | 6.584 | | 1984 | 6.585 | | 1983 | 6.605 | | 1982 | 6.617 | | 1981 | 6.636 | | 1980 | 6.658 | | 1979 | 6.613 | | 1978 | 6.562 | | 1977 | 6.52 | | 1976 | 6.449 | | 1975 | 6.393 | | 1974 | 6.337 | | 1973 | 6.305 | | 1972 | 6.282 | | 1971 | 6.255 | | 1970 | 6.198 | | 1969 | 6.143 | | 1968 | 6.108 | | 1967 | 6.058 | | 1966 | 6.004 | | 1965 | 5.949 | | 1964 | 5.89 | | 1963 | 5.828 | | 1962 | 5.766 | | 1961 | 5.705 | | 1960 | 5.647 |
|