| Year | Value | | 2021 | 2.917 | | 2020 | 2.96 | | 2019 | 3.001 | | 2018 | 3.098 | | 2017 | 3.151 | | 2016 | 3.203 | | 2015 | 3.436 | | 2014 | 3.441 | | 2013 | 3.419 | | 2012 | 3.335 | | 2011 | 3.247 | | 2010 | 3.213 | | 2009 | 3.108 | | 2008 | 3.101 | | 2007 | 3.12 | | 2006 | 3.109 | | 2005 | 3.151 | | 2004 | 3.181 | | 2003 | 3.261 | | 2002 | 3.318 | | 2001 | 3.394 | | 2000 | 3.442 | | 1999 | 3.524 | | 1998 | 3.617 | | 1997 | 3.654 | | 1996 | 3.733 | | 1995 | 3.844 | | 1994 | 3.967 | | 1993 | 4.103 | | 1992 | 4.253 | | 1991 | 4.304 | | 1990 | 4.484 | | 1989 | 4.685 | | 1988 | 4.871 | | 1987 | 4.989 | | 1986 | 5.07 | | 1985 | 5.16 | | 1984 | 5.239 | | 1983 | 5.334 | | 1982 | 5.371 | | 1981 | 5.442 | | 1980 | 5.578 | | 1979 | 5.701 | | 1978 | 5.717 | | 1977 | 5.739 | | 1976 | 5.765 | | 1975 | 5.799 | | 1974 | 5.835 | | 1973 | 5.888 | | 1972 | 5.971 | | 1971 | 6.031 | | 1970 | 6.119 | | 1969 | 6.195 | | 1968 | 6.286 | | 1967 | 6.357 | | 1966 | 6.433 | | 1965 | 6.518 | | 1964 | 6.643 | | 1963 | 6.718 | | 1962 | 6.755 | | 1961 | 6.779 | | 1960 | 6.794 |
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