| Year | Value | | 2021 | 2.469 | | 2020 | 2.497 | | 2019 | 2.525 | | 2018 | 2.561 | | 2017 | 2.591 | | 2016 | 2.624 | | 2015 | 2.645 | | 2014 | 2.646 | | 2013 | 2.636 | | 2012 | 2.625 | | 2011 | 2.675 | | 2010 | 2.73 | | 2009 | 2.787 | | 2008 | 2.846 | | 2007 | 2.907 | | 2006 | 2.97 | | 2005 | 3.04 | | 2004 | 3.119 | | 2003 | 3.21 | | 2002 | 3.315 | | 2001 | 3.431 | | 2000 | 3.553 | | 1999 | 3.678 | | 1998 | 3.798 | | 1997 | 3.911 | | 1996 | 4.014 | | 1995 | 4.108 | | 1994 | 4.196 | | 1993 | 4.282 | | 1992 | 4.368 | | 1991 | 4.457 | | 1990 | 4.547 | | 1989 | 4.639 | | 1988 | 4.73 | | 1987 | 4.818 | | 1986 | 4.9 | | 1985 | 4.974 | | 1984 | 5.037 | | 1983 | 5.087 | | 1982 | 5.126 | | 1981 | 5.165 | | 1980 | 5.122 | | 1979 | 5.089 | | 1978 | 5.064 | | 1977 | 5.117 | | 1976 | 5.154 | | 1975 | 5.207 | | 1974 | 5.272 | | 1973 | 5.351 | | 1972 | 5.458 | | 1971 | 5.591 | | 1970 | 5.739 | | 1969 | 5.87 | | 1968 | 6.01 | | 1967 | 6.134 | | 1966 | 6.258 | | 1965 | 6.356 | | 1964 | 6.415 | | 1963 | 6.447 | | 1962 | 6.472 | | 1961 | 6.489 | | 1960 | 6.5 |
|