# year    value 
 1980 20.35009 
 1981 20.17459 
 1982 19.86266 
 1983 20.44707 
 1984 19.63855 
 1985 19.98733 
 1986 19.84392 
 1987 19.95031 
 1988 19.68665 
 1989 19.45518 
 1990  19.7053 
 1991 20.46238 
 1992 20.42511 
 1993 20.19191 
 1994 20.40805 
 1995 19.95997 
 1996 19.98081 
 1997 20.45505 
 1998 20.43583 
 1999  19.4044 
 2000 19.72006 
 2001  19.6865 
 2002 20.18869 
 2003  20.0147 
 2004 19.94737 
 2005 19.93885 
 2006 19.90422 
 2007 19.48298 
 2008 19.48886 
 2009 20.57006 
 2010 20.23411 
 2011 19.84068 
 2012 20.18362 
 2013 19.94894 
 2014 19.90318 
 2015 20.75184 
 2016 20.02966 
 2017 20.23238 
 2018 20.51689