# year    value 
 1980 19.18713 
 1981 19.04878 
 1982 19.33373 
 1983 19.41988 
 1984  19.3723 
 1985 19.33419 
 1986 19.30981 
 1987 19.72509 
 1988 19.38879 
 1989 19.21505 
 1990 19.51187 
 1991 19.35213 
 1992 19.27769 
 1993  19.4689 
 1994 19.73159 
 1995  19.7373 
 1996 19.38328 
 1997 19.94758 
 1998 19.60759 
 1999 19.21726 
 2000 19.41692 
 2001 19.47873 
 2002 19.82603 
 2003 19.83898 
 2004 19.83129 
 2005 19.59469 
 2006 19.58637 
 2007 19.67941 
 2008  19.7345 
 2009 19.89909 
 2010 19.69486 
 2011 19.51518 
 2012 19.91715 
 2013 19.67168 
 2014 19.96408 
 2015 20.12084 
 2016  19.8138 
 2017 19.83027 
 2018 19.86786