# year    value 
 1980 20.63585 
 1981 20.78001 
 1982 20.55498 
 1983 20.86461 
 1984 20.65463 
 1985 20.54316 
 1986  20.6047 
 1987 20.90767 
 1988 21.03794 
 1989 20.78092 
 1990 20.90316 
 1991 21.02203 
 1992 20.41106 
 1993 20.76382 
 1994 20.87147 
 1995 21.19237 
 1996  20.8617 
 1997 21.01544 
 1998 21.52471 
 1999 21.03637 
 2000 20.99374 
 2001 21.18307 
 2002 21.26378 
 2003 21.23435 
 2004 20.94126 
 2005 21.37822 
 2006 21.36604 
 2007 21.18444 
 2008 21.20148 
 2009 21.22328 
 2010 21.49984 
 2011 21.38928 
 2012 21.46224 
 2013 21.31663 
 2014 21.34864 
 2015 21.50454 
 2016  21.6367 
 2017 21.49898 
 2018  21.3613