| Year | Value | | 2021 | 2.569 | | 2020 | 2.593 | | 2019 | 2.623 | | 2018 | 2.671 | | 2017 | 2.805 | | 2016 | 2.952 | | 2015 | 2.922 | | 2014 | 2.943 | | 2013 | 2.874 | | 2012 | 3.115 | | 2011 | 2.865 | | 2010 | 2.973 | | 2009 | 2.987 | | 2008 | 3.02 | | 2007 | 3.034 | | 2006 | 2.942 | | 2005 | 2.763 | | 2004 | 2.913 | | 2003 | 2.782 | | 2002 | 2.706 | | 2001 | 2.957 | | 2000 | 3.007 | | 1999 | 3.026 | | 1998 | 3.045 | | 1997 | 3.035 | | 1996 | 3.033 | | 1995 | 3.024 | | 1994 | 3.026 | | 1993 | 3.027 | | 1992 | 3.034 | | 1991 | 3.041 | | 1990 | 3.048 | | 1989 | 3.065 | | 1988 | 3.083 | | 1987 | 3.139 | | 1986 | 3.189 | | 1985 | 3.143 | | 1984 | 3.059 | | 1983 | 3.219 | | 1982 | 3.081 | | 1981 | 3.156 | | 1980 | 3.248 | | 1979 | 3.27 | | 1978 | 3.35 | | 1977 | 3.636 | | 1976 | 3.838 | | 1975 | 3.911 | | 1974 | 3.966 | | 1973 | 4.041 | | 1972 | 4.198 | | 1971 | 4.371 | | 1970 | 4.536 | | 1969 | 4.674 | | 1968 | 4.555 | | 1967 | 4.799 | | 1966 | 5.185 | | 1965 | 5.143 | | 1964 | 5.375 | | 1963 | 5.559 | | 1962 | 5.914 | | 1961 | 5.942 | | 1960 | 5.906 |
|