| Year | Value | | 2021 | 1.46 | | 2020 | 1.46 | | 2019 | 1.334 | | 2018 | 1.356 | | 2017 | 1.419 | | 2016 | 1.438 | | 2015 | 1.486 | | 2014 | 1.545 | | 2013 | 1.602 | | 2012 | 1.666 | | 2011 | 1.724 | | 2010 | 1.79 | | 2009 | 1.852 | | 2008 | 1.917 | | 2007 | 1.99 | | 2006 | 2.072 | | 2005 | 2.204 | | 2004 | 2.384 | | 2003 | 2.491 | | 2002 | 2.54 | | 2001 | 2.621 | | 2000 | 2.733 | | 1999 | 2.793 | | 1998 | 2.907 | | 1997 | 3.062 | | 1996 | 3.232 | | 1995 | 3.274 | | 1994 | 3.765 | | 1993 | 3.817 | | 1992 | 4.09 | | 1991 | 4.252 | | 1990 | 4.537 | | 1989 | 4.672 | | 1988 | 4.785 | | 1987 | 4.9 | | 1986 | 5.008 | | 1985 | 5.126 | | 1984 | 5.306 | | 1983 | 5.481 | | 1982 | 5.651 | | 1981 | 5.812 | | 1980 | 5.948 | | 1979 | 6.03 | | 1978 | 6.093 | | 1977 | 6.155 | | 1976 | 6.208 | | 1975 | 6.258 | | 1974 | 6.331 | | 1973 | 6.4 | | 1972 | 6.45 | | 1971 | 6.469 | | 1970 | 6.486 | | 1969 | 6.485 | | 1968 | 6.497 | | 1967 | 6.492 | | 1966 | 6.494 | | 1965 | 6.522 | | 1964 | 6.573 | | 1963 | 6.619 | | 1962 | 6.659 | | 1961 | 6.678 | | 1960 | 6.718 |
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