# year    value 
 1980  19.6286 
 1981 19.70985 
 1982 19.52524 
 1983 19.77882 
 1984 19.57173 
 1985 19.51015 
 1986 19.59499 
 1987 19.82194 
 1988 19.95172 
 1989 19.72981 
 1990 19.87816 
 1991 19.96764 
 1992  19.3939 
 1993 19.68545 
 1994 19.89818 
 1995 20.10206 
 1996 19.79365 
 1997 19.99082 
 1998 20.46212 
 1999 19.97742 
 2000 19.98149 
 2001 20.16124 
 2002 20.23566 
 2003 20.19114 
 2004 19.92782 
 2005 20.32582 
 2006 20.33943 
 2007 20.17345 
 2008 20.16777 
 2009 20.19088 
 2010 20.43584 
 2011 20.34476 
 2012 20.41023 
 2013 20.29832 
 2014  20.3248 
 2015 20.49701 
 2016 20.60259 
 2017 20.49595 
 2018 20.36764