# year    value 
 1980 20.08006 
 1981 19.85738 
 1982 19.70117 
 1983 19.77393 
 1984 19.67972 
 1985 19.69029 
 1986 19.70836 
 1987 20.18522 
 1988  20.1486 
 1989 19.74715 
 1990 20.08812 
 1991 20.00678 
 1992 19.64232 
 1993 19.96354 
 1994 20.16628 
 1995 20.17333 
 1996 20.00769 
 1997 19.92471 
 1998 20.68081 
 1999 20.30185 
 2000 20.08031 
 2001 20.31205 
 2002  20.4216 
 2003 20.44543 
 2004 20.26825 
 2005 20.48444 
 2006 20.55405 
 2007 20.41667 
 2008 20.25836 
 2009 20.79715 
 2010 20.78769 
 2011 20.37143 
 2012  20.4735 
 2013  20.5439 
 2014 20.58988 
 2015 20.79381 
 2016 21.03454 
 2017 20.89693 
 2018  20.7285