# year    value 
 1980 19.28587 
 1981 18.95408 
 1982 18.77103 
 1983 19.74078 
 1984 18.96095 
 1985 18.97857 
 1986 19.14396 
 1987 19.47221 
 1988 19.22457 
 1989 18.80829 
 1990 19.21458 
 1991 19.33131 
 1992 19.63907 
 1993 19.36532 
 1994 19.35497 
 1995 19.27248 
 1996 19.03337 
 1997 19.15592 
 1998 19.76059 
 1999 19.04864 
 2000 19.08207 
 2001 19.25919 
 2002  19.6236 
 2003 19.42637 
 2004 19.24654 
 2005 19.46849 
 2006 19.15184 
 2007 19.22836 
 2008 18.83262 
 2009 19.43459 
 2010 19.66626 
 2011   19.203 
 2012 19.27395 
 2013 19.17128 
 2014 19.28003 
 2015 19.58675 
 2016 19.59227 
 2017 19.53869 
 2018 19.52727