| Year | Value | | 2017 | 5.17 | | 2016 | 6.38 | | 2015 | 6.17 | | 2014 | 6.15 | | 2013 | 6.78 | | 2012 | 6.89 | | 2011 | 7.31 | | 2010 | 7.48 | | 2009 | 7.28 | | 2008 | 7.54 | | 2007 | 7.49 | | 2006 | 7.85 | | 2005 | 8.09 | | 2004 | 8.46 | | 2003 | 9.06 | | 2002 | 9.77 | | 2001 | 10.05 | | 2000 | 10.76 | | 1999 | 10.77 | | 1998 | 10.18 | | 1997 | 10.31 | | 1996 | 10.13 | | 1995 | 12.24 | | 1994 | 15.58 | | 1993 | 13.36 | | 1992 | 14.11 | | 1991 | 13.07 | | 1990 | 15.02 | | 1989 | 13.76 | | 1988 | 8.88 |
|