| Year | Value | | 2021 | 4.643 | | 2020 | 4.75 | | 2019 | 4.87 | | 2018 | 5.002 | | 2017 | 5.129 | | 2016 | 5.262 | | 2015 | 5.405 | | 2014 | 5.56 | | 2013 | 5.696 | | 2012 | 5.83 | | 2011 | 5.958 | | 2010 | 6.099 | | 2009 | 6.235 | | 2008 | 6.376 | | 2007 | 6.53 | | 2006 | 6.722 | | 2005 | 6.905 | | 2004 | 7.069 | | 2003 | 7.22 | | 2002 | 7.339 | | 2001 | 7.446 | | 2000 | 7.534 | | 1999 | 7.599 | | 1998 | 7.642 | | 1997 | 7.667 | | 1996 | 7.709 | | 1995 | 7.712 | | 1994 | 7.717 | | 1993 | 7.719 | | 1992 | 7.665 | | 1991 | 7.606 | | 1990 | 7.565 | | 1989 | 7.533 | | 1988 | 7.531 | | 1987 | 7.526 | | 1986 | 7.519 | | 1985 | 7.517 | | 1984 | 7.513 | | 1983 | 7.537 | | 1982 | 7.553 | | 1981 | 7.568 | | 1980 | 7.588 | | 1979 | 7.612 | | 1978 | 7.599 | | 1977 | 7.591 | | 1976 | 7.561 | | 1975 | 7.542 | | 1974 | 7.526 | | 1973 | 7.487 | | 1972 | 7.453 | | 1971 | 7.432 | | 1970 | 7.4 | | 1969 | 7.389 | | 1968 | 7.363 | | 1967 | 7.339 | | 1966 | 7.32 | | 1965 | 7.305 | | 1964 | 7.304 | | 1963 | 7.302 | | 1962 | 7.292 | | 1961 | 7.284 | | 1960 | 7.282 |
|