| Year | Value | | 2017 | 7.45 | | 2016 | 7.27 | | 2015 | 7.24 | | 2014 | 7.13 | | 2013 | 6.82 | | 2012 | 6.61 | | 2011 | 6.45 | | 2010 | 6.62 | | 2009 | 6.72 | | 2008 | 6.61 | | 2007 | 6.46 | | 2006 | 6.28 | | 2005 | 6.41 | | 2004 | 6.13 | | 2003 | 6.29 | | 2002 | 6.49 | | 2001 | 7.2 | | 2000 | 7.41 | | 1999 | 7.4899997711 | | 1998 | 8.4099998474 | | 1997 | 10.279999733 | | 1996 | 10.470000267 | | 1995 | 10.3699998856 | | 1994 | 10.196100235 | | 1993 | 10.4941997528 | | 1992 | 10.2063999176 | | 1991 | 9.7681999207 | | 1990 | 9.7908000946 | | 1989 | 9.7020998001099 | | 1988 | 9.5601997375488 | | 1987 | 9.5642004013062 | | 1986 | 9.4520998001099 | | 1985 | 9.1140003204346 | | 1984 | 8.9630002975464 | | 1983 | 8.8767995834351 | | 1982 | 8.8866996765137 | | 1981 | 8.8590002059937 | | 1980 | 8.8506002426147 | | 1960 | 6.213677406311 |
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