| Year | Value | | 2021 | 5.028 | | 2020 | 5.082 | | 2019 | 4.989 | | 2018 | 5.052 | | 2017 | 5.11 | | 2016 | 5.176 | | 2015 | 5.26 | | 2014 | 5.407 | | 2013 | 5.421 | | 2012 | 5.468 | | 2011 | 5.544 | | 2010 | 5.614 | | 2009 | 5.713 | | 2008 | 5.714 | | 2007 | 5.863 | | 2006 | 5.797 | | 2005 | 5.829 | | 2004 | 5.854 | | 2003 | 5.884 | | 2002 | 5.899 | | 2001 | 5.92 | | 2000 | 5.939 | | 1999 | 5.957 | | 1998 | 5.99 | | 1997 | 6.041 | | 1996 | 6.089 | | 1995 | 6.161 | | 1994 | 6.224 | | 1993 | 6.308 | | 1992 | 6.38 | | 1991 | 6.461 | | 1990 | 6.548 | | 1989 | 6.647 | | 1988 | 6.755 | | 1987 | 6.887 | | 1986 | 7.401 | | 1985 | 7.151 | | 1984 | 7.299 | | 1983 | 7.432 | | 1982 | 7.58 | | 1981 | 7.78 | | 1980 | 7.999 | | 1979 | 8.257 | | 1978 | 8.565 | | 1977 | 9.182 | | 1976 | 9.399 | | 1975 | 10.909 | | 1974 | 10.339 | | 1973 | 10.821 | | 1972 | 11.341 | | 1971 | 11.897 | | 1970 | 12.236 | | 1969 | 12.685 | | 1968 | 13.223 | | 1967 | 13.857 | | 1966 | 14.311 | | 1965 | 14.801 | | 1964 | 15.289 | | 1963 | 15.78 | | 1962 | 16.251 | | 1961 | 16.722 | | 1960 | 17.071 |
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