# year    value 
 1980 20.56421 
 1981 20.64839 
 1982  20.4319 
 1983   20.766 
 1984  20.4783 
 1985 20.36136 
 1986 20.42589 
 1987  20.7744 
 1988 20.92629 
 1989  20.6674 
 1990 20.79766 
 1991 20.88863 
 1992 20.22211 
 1993 20.63876 
 1994 20.70843 
 1995 21.07257 
 1996 20.67716 
 1997 20.89504 
 1998 21.42028 
 1999 20.90188 
 2000 20.79778 
 2001 21.04409 
 2002 21.13749 
 2003 21.08917 
 2004 20.78027 
 2005  21.2738 
 2006 21.29459 
 2007  21.1194 
 2008 21.10272 
 2009  21.1354 
 2010 21.53101 
 2011 21.32847 
 2012 21.48479 
 2013 21.25564 
 2014  21.3334 
 2015  21.4959 
 2016 21.68088 
 2017 21.52632 
 2018 21.40639