| Year | Value | | 2018 | 13.012937642089 | | 2017 | 12.468418263459 | | 2016 | 12.629886477932 | | 2015 | 12.528105198246 | | 2014 | 12.044929495184 | | 2013 | 12.293148454158 | | 2012 | 12.072438451155 | | 2011 | 11.975800053435 | | 2010 | 11.950703401877 | | 2009 | 12.247579397983 | | 2008 | 13.090003839983 | | 1999 | 11.487133766608 | | 1998 | 11.587173757501 | | 1997 | 12.837610481517 | | 1996 | 12.062636013825 | | 1995 | 11.792383145185 | | 1994 | 11.344990422864 | | 1993 | 10.862001547946 | | 1992 | 11.871330294636 | | 1991 | 11.694936518318 | | 1990 | 11.704661448 | | 1989 | 12.023458565097 |
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