| Year | Value | | 2021 | 5.117 | | 2020 | 4.99 | | 2019 | 5.033 | | 2018 | 5.554 | | 2017 | 6.033 | | 2016 | 6.999 | | 2015 | 7.297 | | 2014 | 8.072 | | 2013 | 7.696 | | 2012 | 6.417 | | 2011 | 3.934 | | 2010 | 3.701 | | 2009 | 3.678 | | 2008 | 3.721 | | 2007 | 3.67 | | 2006 | 3.775 | | 2005 | 3.924 | | 2004 | 3.986 | | 2003 | 3.971 | | 2002 | 4.091 | | 2001 | 4.157 | | 2000 | 4.413 | | 1999 | 4.509 | | 1998 | 4.575 | | 1997 | 4.54 | | 1996 | 4.521 | | 1995 | 4.503 | | 1994 | 4.599 | | 1993 | 4.588 | | 1992 | 4.574 | | 1991 | 4.67 | | 1990 | 4.784 | | 1989 | 4.851 | | 1988 | 4.94 | | 1987 | 5.124 | | 1986 | 5.215 | | 1985 | 5.424 | | 1984 | 5.44 | | 1983 | 5.877 | | 1982 | 8.709 | | 1981 | 7.425 | | 1980 | 6.732 | | 1979 | 6.932 | | 1978 | 7.177 | | 1977 | 8.054 | | 1976 | 8.131 | | 1975 | 8.418 | | 1974 | 8.733 | | 1973 | 9.796 | | 1972 | 9.574 | | 1971 | 9.984 | | 1970 | 10.54 | | 1969 | 11.006 | | 1968 | 11.547 | | 1967 | 12.172 | | 1966 | 12.628 | | 1965 | 13.081 | | 1964 | 13.602 | | 1963 | 14.211 | | 1962 | 14.843 | | 1961 | 15.485 | | 1960 | 16.314 |
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