# year    value 
 1980 20.23221 
 1981  20.6798 
 1982 20.01779 
 1983 20.63358 
 1984 20.40684 
 1985 20.03415 
 1986 20.17864 
 1987 20.29715 
 1988 21.01183 
 1989 20.63886 
 1990 20.55292 
 1991 20.82305 
 1992 19.61846 
 1993 19.99482 
 1994 20.72504 
 1995  20.9731 
 1996 20.53698 
 1997 20.61516 
 1998 21.28129 
 1999 21.05505 
 2000   20.859 
 2001 21.00859 
 2002   20.973 
 2003 20.91266 
 2004  20.4002 
 2005 21.12232 
 2006 21.32498 
 2007 21.22102 
 2008 21.14528 
 2009 20.55912 
 2010 21.71577 
 2011  21.3547 
 2012 21.77856 
 2013  21.0886 
 2014 21.00078 
 2015 21.25051 
 2016 21.57931 
 2017 21.24522 
 2018 21.53712