# year    value 
 1980 20.35873 
 1981 20.00179 
 1982 20.23094 
 1983 19.73102 
 1984  19.8726 
 1985 20.24632 
 1986 20.19087 
 1987 20.35283 
 1988 20.59526 
 1989 19.58952 
 1990 20.31027 
 1991 20.73987 
 1992 19.69382 
 1993 20.08584 
 1994 20.42265 
 1995 20.10905 
 1996 20.67694 
 1997 20.35432 
 1998 20.88074 
 1999 20.89922 
 2000 20.34367 
 2001  20.5125 
 2002 20.77295 
 2003 20.99264 
 2004 20.79947 
 2005 20.84549 
 2006 21.18497 
 2007 20.56884 
 2008 20.28755 
 2009 21.11209 
 2010 21.47178 
 2011 20.78333 
 2012 20.41991 
 2013 20.98821 
 2014 20.52909 
 2015 20.65617 
 2016 20.83855 
 2017 21.07622 
 2018 20.88101