# year    value 
 1980 19.90731 
 1981 20.16923 
 1982 19.96111 
 1983  20.2746 
 1984 19.93881 
 1985 19.73214 
 1986 19.57288 
 1987 19.70293 
 1988 19.69186 
 1989 20.26037 
 1990   20.113 
 1991 20.01183 
 1992 19.20934 
 1993 19.55781 
 1994 19.41507 
 1995 20.05673 
 1996 19.51366 
 1997 19.94794 
 1998 19.60914 
 1999 20.16673 
 2000 19.69505 
 2001 20.49824 
 2002 19.27039 
 2003 20.11647 
 2004 20.05072 
 2005 19.97079 
 2006 20.07207 
 2007 20.18051 
 2008  20.1037 
 2009 20.40496 
 2010 20.17312 
 2011 20.05781 
 2012 20.16081 
 2013 20.48804 
 2014 20.55635 
 2015 20.19031 
 2016 20.56094 
 2017  20.5097 
 2018 20.79601