# year    value 
 1980  16.5134 
 1981 17.19721 
 1982  18.2081 
 1983 18.75235 
 1984  16.7488 
 1985 17.39682 
 1986 17.49543 
 1987 17.24708 
 1988 17.88505 
 1989 17.52635 
 1990 17.90496 
 1991 18.26812 
 1992 18.77695 
 1993 17.91642 
 1994  19.4662 
 1995  18.0918 
 1996 17.57867 
 1997 17.85102 
 1998 18.70535 
 1999 18.22973 
 2000 18.48262 
 2001 18.38709 
 2002 18.75902 
 2003 20.44497 
 2004 18.15999 
 2005 18.13783 
 2006 18.67956 
 2007 18.60373 
 2008  18.6492 
 2009 18.92568 
 2010  18.8348 
 2011 18.43309 
 2012  19.5793 
 2013 19.17872 
 2014 18.08055 
 2015 20.38657 
 2016 18.94755 
 2017 20.13903 
 2018 20.13272