| Year | Value | | 2022 | 4.91 | | 2021 | 6.34 | | 2020 | 7.23 | | 2019 | 7.54 | | 2018 | 7.83 | | 2017 | 7.89 | | 2016 | 6.45 | | 2015 | 7.38 | | 2014 | 6.61 | | 2013 | 6.08 | | 2012 | 5.19 | | 2011 | 5.06 | | 2010 | 4.37 | | 2009 | 4.02 | | 2006 | 1.63 | | 2005 | 2.11 | | 2000 | 2.8 | | 1999 | 3 | | 1998 | 4.1 | | 1997 | 4.4 | | 1996 | 5.1 | | 1995 | 3.9 | | 1994 | 3.9 | | 1993 | 5.1 | | 1992 | 4.2 | | 1991 | 4.3 | | 1990 | 5.5 | | 1989 | 5.3 | | 1988 | 6 | | 1987 | 6.2 | | 1986 | 6.9 | | 1985 | 6.9 | | 1984 | 7.2 | | 1980 | 5.8 | | 1979 | 4.8 |
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