| Year | Value | | 2021 | 3.186 | | 2020 | 3.237 | | 2019 | 3.281 | | 2018 | 3.294 | | 2017 | 3.29 | | 2016 | 3.384 | | 2015 | 3.459 | | 2014 | 3.459 | | 2013 | 3.433 | | 2012 | 3.421 | | 2011 | 3.456 | | 2010 | 3.457 | | 2009 | 3.43 | | 2008 | 3.388 | | 2007 | 3.367 | | 2006 | 3.385 | | 2005 | 3.438 | | 2004 | 3.529 | | 2003 | 3.606 | | 2002 | 3.767 | | 2001 | 3.884 | | 2000 | 4.006 | | 1999 | 4.127 | | 1998 | 4.281 | | 1997 | 4.317 | | 1996 | 4.441 | | 1995 | 4.658 | | 1994 | 4.766 | | 1993 | 4.812 | | 1992 | 4.965 | | 1991 | 5.176 | | 1990 | 5.342 | | 1989 | 5.46 | | 1988 | 5.57 | | 1987 | 5.658 | | 1986 | 5.734 | | 1985 | 5.773 | | 1984 | 5.803 | | 1983 | 5.853 | | 1982 | 5.902 | | 1981 | 6.007 | | 1980 | 6.11 | | 1979 | 6.206 | | 1978 | 6.304 | | 1977 | 6.389 | | 1976 | 6.493 | | 1975 | 6.6 | | 1974 | 6.675 | | 1973 | 6.75 | | 1972 | 6.845 | | 1971 | 6.943 | | 1970 | 7.003 | | 1969 | 7.031 | | 1968 | 7.035 | | 1967 | 7.024 | | 1966 | 6.999 | | 1965 | 6.962 | | 1964 | 6.911 | | 1963 | 6.844 | | 1962 | 6.76 | | 1961 | 6.661 | | 1960 | 6.547 |
|