| Year | Value | | 2021 | 3.823 | | 2020 | 3.893 | | 2019 | 3.964 | | 2018 | 4.044 | | 2017 | 4.142 | | 2016 | 4.274 | | 2015 | 4.387 | | 2014 | 4.511 | | 2013 | 4.6 | | 2012 | 4.667 | | 2011 | 4.716 | | 2010 | 4.727 | | 2009 | 4.776 | | 2008 | 4.825 | | 2007 | 4.872 | | 2006 | 4.919 | | 2005 | 4.964 | | 2004 | 5.007 | | 2003 | 5.049 | | 2002 | 5.09 | | 2001 | 5.131 | | 2000 | 5.176 | | 1999 | 5.226 | | 1998 | 5.282 | | 1997 | 5.345 | | 1996 | 5.414 | | 1995 | 5.487 | | 1994 | 5.562 | | 1993 | 5.635 | | 1992 | 5.704 | | 1991 | 5.769 | | 1990 | 5.828 | | 1989 | 5.884 | | 1988 | 5.938 | | 1987 | 5.992 | | 1986 | 6.046 | | 1985 | 6.101 | | 1984 | 6.158 | | 1983 | 6.217 | | 1982 | 6.276 | | 1981 | 6.333 | | 1980 | 6.386 | | 1979 | 6.434 | | 1978 | 6.473 | | 1977 | 6.504 | | 1976 | 6.524 | | 1975 | 6.534 | | 1974 | 6.534 | | 1973 | 6.525 | | 1972 | 6.51 | | 1971 | 6.49 | | 1970 | 6.468 | | 1969 | 6.444 | | 1968 | 6.419 | | 1967 | 6.394 | | 1966 | 6.371 | | 1965 | 6.349 | | 1964 | 6.327 | | 1963 | 6.306 | | 1962 | 6.284 | | 1961 | 6.263 | | 1960 | 6.242 |
|