| Year | Value | | 2022 | 6.755 | | 2021 | 7.311 | | 2020 | 7.548 | | 2019 | 6.595 | | 2018 | 6.217 | | 2017 | 6.138 | | 2016 | 5.255 | | 2015 | 4.556 | | 2014 | 5.034 | | 2013 | 4.385 | | 2012 | 4.086 | | 2011 | 4.67 | | 2010 | 4.569 | | 2009 | 5.46 | | 2008 | 4.415 | | 2007 | 4.713 | | 2006 | 5.277 | | 2005 | 4.825 | | 2004 | 6.513 | | 2003 | 6.814 | | 2002 | 5.54 | | 2001 | 6.206 | | 2000 | 7.61 | | 1999 | 5.343 | | 1998 | 4.417 | | 1997 | 5.36 | | 1996 | 8.15 | | 1995 | 3.4 | | 1994 | 4.41 | | 1993 | 5.06 | | 1992 | 4.98 | | 1991 | 5.1 | | 1990 | 5.8 | | 1989 | 5.62 | | 1988 | 4.72 | | 1987 | 5.51 | | 1986 | 6.1 | | 1985 | 5.13 | | 1984 | 7.33 | | 1983 | 8.25 | | 1982 | 5.61 | | 1980 | 4.11 | | 1979 | 5.69 |
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