| Year | Value | | 2021 | 3.821 | | 2020 | 3.873 | | 2019 | 3.939 | | 2018 | 3.994 | | 2017 | 4.027 | | 2016 | 4.06 | | 2015 | 4.081 | | 2014 | 4.112 | | 2013 | 4.159 | | 2012 | 4.233 | | 2011 | 4.357 | | 2010 | 4.532 | | 2009 | 4.716 | | 2008 | 4.901 | | 2007 | 5.1 | | 2006 | 5.28 | | 2005 | 5.436 | | 2004 | 5.573 | | 2003 | 5.674 | | 2002 | 5.752 | | 2001 | 5.844 | | 2000 | 5.924 | | 1999 | 6.013 | | 1998 | 6.116 | | 1997 | 6.176 | | 1996 | 6.251 | | 1995 | 6.324 | | 1994 | 6.408 | | 1993 | 6.51 | | 1992 | 6.607 | | 1991 | 6.705 | | 1990 | 6.873 | | 1989 | 7.058 | | 1988 | 7.227 | | 1987 | 7.39 | | 1986 | 7.559 | | 1985 | 7.725 | | 1984 | 7.872 | | 1983 | 7.987 | | 1982 | 8.085 | | 1981 | 8.156 | | 1980 | 8.189 | | 1979 | 8.23 | | 1978 | 8.25 | | 1977 | 8.24 | | 1976 | 8.243 | | 1975 | 8.224 | | 1974 | 8.231 | | 1973 | 8.232 | | 1972 | 8.233 | | 1971 | 8.228 | | 1970 | 8.231 | | 1969 | 8.217 | | 1968 | 8.207 | | 1967 | 8.201 | | 1966 | 8.198 | | 1965 | 8.198 | | 1964 | 8.198 | | 1963 | 8.198 | | 1962 | 8.197 | | 1961 | 8.194 | | 1960 | 8.187 |
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