| Year | Value | | 2017 | 5.503 | | 2016 | 5.453 | | 2015 | 5.41 | | 2014 | 5.373 | | 2013 | 5.339 | | 2012 | 5.307 | | 2011 | 5.277 | | 2010 | 5.247 | | 2009 | 5.218 | | 2008 | 5.191 | | 2007 | 5.169 | | 2006 | 5.152 | | 2005 | 5.14 | | 2004 | 5.135 | | 2003 | 5.136 | | 2002 | 5.145 | | 2001 | 5.161 | | 2000 | 5.189 | | 1999 | 5.231 | | 1998 | 5.288 | | 1997 | 5.357 | | 1996 | 5.434 | | 1995 | 5.511 | | 1994 | 5.58 | | 1993 | 5.635 | | 1992 | 5.672 | | 1991 | 5.693 | | 1990 | 5.698 | | 1989 | 5.695 | | 1988 | 5.692 | | 1987 | 5.698 | | 1986 | 5.718 | | 1985 | 5.758 | | 1984 | 5.823 | | 1983 | 5.909 | | 1982 | 6.017 | | 1981 | 6.143 | | 1980 | 6.286 | | 1979 | 6.441 | | 1978 | 6.602 | | 1977 | 6.768 | | 1976 | 6.941 | | 1975 | 7.122 | | 1974 | 7.318 | | 1973 | 7.534 | | 1972 | 7.774 | | 1971 | 8.037 | | 1970 | 8.322 | | 1969 | 8.626 | | 1968 | 8.945 | | 1967 | 9.273 | | 1966 | 9.608 | | 1965 | 9.945 | | 1964 | 10.282 | | 1963 | 10.619 | | 1962 | 10.959 | | 1961 | 11.306 | | 1960 | 11.666 |
|