# year    value 
 1980 18.81523 
 1981  19.0677 
 1982 18.54212 
 1983 18.91288 
 1984 18.81947 
 1985 18.68222 
 1986 18.75901 
 1987 18.73726 
 1988 19.40129 
 1989 19.02425 
 1990 19.11134 
 1991 19.24824 
 1992 18.21392 
 1993  18.5336 
 1994 19.37376 
 1995 19.36916 
 1996 19.04391 
 1997 19.15344 
 1998 19.79961 
 1999 19.51976 
 2000 19.48438 
 2001 19.66047 
 2002 19.60283 
 2003  19.5096 
 2004 19.03887 
 2005 19.64458 
 2006 19.90181 
 2007 19.81149 
 2008 19.74541 
 2009 19.18571 
 2010 20.23733 
 2011 19.88529 
 2012 20.24382 
 2013 19.71764 
 2014 19.63318 
 2015 19.88981 
 2016 20.16528 
 2017 20.01175 
 2018  20.1483