| Year | Value | | 2021 | 2.175 | | 2020 | 2.194 | | 2019 | 2.215 | | 2018 | 2.234 | | 2017 | 2.262 | | 2016 | 2.309 | | 2015 | 2.345 | | 2014 | 2.389 | | 2013 | 2.427 | | 2012 | 2.49 | | 2011 | 2.499 | | 2010 | 2.452 | | 2009 | 2.459 | | 2008 | 2.479 | | 2007 | 2.491 | | 2006 | 2.453 | | 2005 | 2.433 | | 2004 | 2.418 | | 2003 | 2.428 | | 2002 | 2.456 | | 2001 | 2.497 | | 2000 | 2.536 | | 1999 | 2.578 | | 1998 | 2.655 | | 1997 | 2.738 | | 1996 | 2.77 | | 1995 | 2.798 | | 1994 | 2.838 | | 1993 | 2.882 | | 1992 | 2.942 | | 1991 | 3.056 | | 1990 | 3.098 | | 1989 | 3.223 | | 1988 | 3.331 | | 1987 | 3.42 | | 1986 | 3.527 | | 1985 | 3.711 | | 1984 | 3.941 | | 1983 | 4.095 | | 1982 | 4.249 | | 1981 | 4.359 | | 1980 | 4.493 | | 1979 | 4.613 | | 1978 | 4.715 | | 1977 | 4.809 | | 1976 | 4.923 | | 1975 | 5.035 | | 1974 | 5.093 | | 1973 | 5.217 | | 1972 | 5.291 | | 1971 | 5.363 | | 1970 | 5.447 | | 1969 | 5.506 | | 1968 | 5.54 | | 1967 | 5.577 | | 1966 | 5.601 | | 1965 | 5.616 | | 1964 | 5.614 | | 1963 | 5.604 | | 1962 | 5.589 | | 1961 | 5.57 | | 1960 | 5.547 |
|