| Year | Value | | 2021 | 8.271 | | 2020 | 6.253 | | 2019 | 5.178 | | 2018 | 4.704 | | 2017 | 4.495 | | 2016 | 4.343 | | 2015 | 4.235 | | 2014 | 4.221 | | 2013 | 4.197 | | 2012 | 4.134 | | 2011 | 4.099 | | 2010 | 4.066 | | 2009 | 4.039 | | 2008 | 4.026 | | 2007 | 4.093 | | 2006 | 4.333 | | 2005 | 4.16 | | 2004 | 4.158 | | 2003 | 4.196 | | 2002 | 4.295 | | 2001 | 4.473 | | 2000 | 4.554 | | 1999 | 4.647 | | 1998 | 4.788 | | 1997 | 4.787 | | 1996 | 4.92 | | 1995 | 4.969 | | 1994 | 5.068 | | 1993 | 5.153 | | 1992 | 5.207 | | 1991 | 5.199 | | 1990 | 7.848 | | 1989 | 7.963 | | 1988 | 9.128 | | 1987 | 9.278 | | 1986 | 9.432 | | 1985 | 9.672 | | 1984 | 9.766 | | 1983 | 9.941 | | 1982 | 19.378 | | 1981 | 10.22 | | 1980 | 10.374 | | 1979 | 10.505 | | 1978 | 10.596 | | 1977 | 10.689 | | 1976 | 31.168 | | 1975 | 11.148 | | 1974 | 7.41 | | 1973 | 7.486 | | 1972 | 7.677 | | 1971 | 7.772 | | 1970 | 7.867 | | 1969 | 7.968 | | 1968 | 8.091 | | 1967 | 8.224 | | 1966 | 8.485 | | 1965 | 8.642 | | 1964 | 8.818 | | 1963 | 9.073 | | 1962 | 9.321 | | 1961 | 9.5 | | 1960 | 9.837 |
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