| Year | Value | | 2022 | 5.101 | | 2021 | 6.398 | | 2020 | 6.378 | | 2019 | 5.084 | | 2018 | 5.065 | | 2017 | 5.129 | | 2016 | 5.234 | | 2015 | 5.125 | | 2014 | 5.046 | | 2013 | 5.04 | | 2012 | 5.205 | | 2011 | 5.44 | | 2010 | 5.251 | | 2009 | 5.269 | | 2008 | 5.292 | | 2007 | 5.288 | | 2006 | 5.33 | | 2005 | 5.35 | | 2004 | 5.367 | | 2003 | 5.413 | | 2002 | 5.464 | | 2001 | 5.551 | | 2000 | 5.616 | | 1999 | 6.002 | | 1998 | 5.832 | | 1997 | 5.963 | | 1996 | 6.081 | | 1995 | 6.229 | | 1994 | 6.308 | | 1993 | 6.414 | | 1992 | 6.546 | | 1991 | 6.664 | | 1990 | 6.841 | | 1989 | 7.046 | | 1988 | 7.244 | | 1987 | 7.484 | | 1986 | 7.737 | | 1985 | 8.015 | | 1984 | 8.314 | | 1983 | 8.643 | | 1982 | 9 | | 1981 | 9.393 | | 1980 | 9.754 | | 1979 | 10.129 | | 1978 | 10.478 | | 1977 | 10.895 | | 1976 | 11.317 | | 1975 | 11.617 | | 1974 | 11.904 |
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