| Year | Value | | 2019 | 4.767 | | 2018 | 4.836 | | 2017 | 4.906 | | 2016 | 4.977 | | 2015 | 5.047 | | 2014 | 5.116 | | 2013 | 5.183 | | 2012 | 5.246 | | 2011 | 5.305 | | 2010 | 5.362 | | 2009 | 5.416 | | 2008 | 5.469 | | 2007 | 5.522 | | 2006 | 5.576 | | 2005 | 5.632 | | 2004 | 5.691 | | 2003 | 5.754 | | 2002 | 5.82 | | 2001 | 5.889 | | 2000 | 5.962 | | 1999 | 6.038 | | 1998 | 6.118 | | 1997 | 6.199 | | 1996 | 6.282 | | 1995 | 6.364 | | 1994 | 6.446 | | 1993 | 6.526 | | 1992 | 6.603 | | 1991 | 6.677 | | 1990 | 6.744 | | 1989 | 6.805 | | 1988 | 6.858 | | 1987 | 6.904 | | 1986 | 6.942 | | 1985 | 6.972 | | 1984 | 6.994 | | 1983 | 7.01 | | 1982 | 7.021 | | 1981 | 7.025 | | 1980 | 7.025 | | 1979 | 7.018 | | 1978 | 7.004 | | 1977 | 6.984 | | 1976 | 6.959 | | 1975 | 6.929 | | 1974 | 6.896 | | 1973 | 6.86 | | 1972 | 6.823 | | 1971 | 6.786 | | 1970 | 6.748 | | 1969 | 6.71 | | 1968 | 6.671 | | 1967 | 6.63 | | 1966 | 6.588 | | 1965 | 6.544 | | 1964 | 6.497 | | 1963 | 6.447 | | 1962 | 6.394 | | 1961 | 6.339 | | 1960 | 6.282 |
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