# year    value 
 1980 20.28589 
 1981 20.06266 
 1982 20.13376 
 1983 20.36791 
 1984 19.88568 
 1985 20.09601 
 1986 19.94731 
 1987 20.54008 
 1988 20.12255 
 1989  20.2631 
 1990  20.4798 
 1991 20.00659 
 1992 19.89659 
 1993 20.27149 
 1994 20.25818 
 1995 20.46261 
 1996 20.35309 
 1997 20.42501 
 1998 20.90075 
 1999 20.43378 
 2000 20.26649 
 2001 20.58589 
 2002 20.53517 
 2003 20.68189 
 2004  20.4395 
 2005 20.76032 
 2006 20.71794 
 2007 20.47602 
 2008 20.48731 
 2009 20.88672 
 2010 20.71593 
 2011 20.65058 
 2012 20.67762 
 2013 20.52207 
 2014  20.7453 
 2015 21.19757 
 2016 21.07351 
 2017 20.97143 
 2018 20.56745