# year    value 
 1980 20.33823 
 1981 20.21846 
 1982  20.7758 
 1983 19.97333 
 1984 19.83879 
 1985 20.47581 
 1986 20.84304 
 1987 21.08764 
 1988 20.22563 
 1989 20.57723 
 1990 20.37768 
 1991 20.58336 
 1992 20.52239 
 1993 20.61019 
 1994 21.01381 
 1995 20.83509 
 1996 20.45319 
 1997 21.39925 
 1998 21.21296 
 1999 20.23647 
 2000 20.29865 
 2001   20.821 
 2002  20.5825 
 2003 20.57478 
 2004 20.77176 
 2005 21.01449 
 2006 21.25265 
 2007 19.87108 
 2008 20.90453 
 2009 20.85741 
 2010 20.53267 
 2011 20.88097 
 2012 20.87533 
 2013 20.66807 
 2014  21.3808 
 2015 21.84227 
 2016 20.93907 
 2017 21.42461 
 2018 20.17289