# year    value 
 1980 19.80901 
 1981 20.09379 
 1982  19.8136 
 1983 20.10732 
 1984 19.77456 
 1985 19.49963 
 1986 19.41559 
 1987 19.73433 
 1988 19.64529 
 1989 20.15251 
 1990 19.97837 
 1991 19.91907 
 1992 19.13437 
 1993 19.52839 
 1994 19.38262 
 1995 19.92808 
 1996 19.48292 
 1997 19.74338 
 1998 19.62326 
 1999 20.21856 
 2000 19.57255 
 2001 20.32549 
 2002 19.27161 
 2003  20.0404 
 2004 19.99964 
 2005 19.94149 
 2006 20.04578 
 2007 20.02459 
 2008  20.0884 
 2009 20.34698 
 2010 20.00356 
 2011 20.07683 
 2012 20.20078 
 2013 20.37158 
 2014 20.45867 
 2015 20.11106 
 2016 20.59249 
 2017 20.53527 
 2018 20.73553