# year    value 
 1980 17.04706 
 1981 17.39232 
 1982  16.9329 
 1983 17.08858 
 1984 17.10122 
 1985 17.06519 
 1986 17.04738 
 1987 17.06875 
 1988 17.36628 
 1989 16.90767 
 1990 17.27029 
 1991 17.28845 
 1992 16.68988 
 1993 16.73936 
 1994 17.40812 
 1995 17.33418 
 1996 16.94864 
 1997 17.26647 
 1998 17.56842 
 1999 17.41011 
 2000 17.42686 
 2001 17.52915 
 2002 17.66123 
 2003   17.311 
 2004 17.18298 
 2005 17.71116 
 2006 17.87404 
 2007 17.64308 
 2008 17.44134 
 2009 17.67883 
 2010 17.82149 
 2011  17.7014 
 2012 17.74357 
 2013 17.93483 
 2014  17.5676 
 2015 17.47964 
 2016 18.02393 
 2017 17.82614 
 2018 18.06428