# year    value 
 1979 18.22214 
 1980 18.37123 
 1981 18.21663 
 1982 18.34031 
 1983 18.70479 
 1984 18.22103 
 1985 18.11902 
 1986 18.26603 
 1987 18.70043 
 1988 18.52925 
 1989 18.23154 
 1990 18.52615 
 1991 18.48719 
 1992 18.41773 
 1993  18.5646 
 1994 18.63842 
 1995 18.76517 
 1996 18.46172 
 1997 18.67624 
 1998 19.14203 
 1999 18.40496 
 2000 18.38869 
 2001 18.58726 
 2002 18.79684 
 2003 18.88919 
 2004 18.67335 
 2005 18.93483 
 2006 18.77723 
 2007 18.79073 
 2008 18.59109 
 2009 18.90243 
 2010 19.09972 
 2011 18.66322 
 2012 18.90719 
 2013 18.90929 
 2014 19.03422 
 2015 19.34409 
 2016 19.41055 
 2017 19.17077 
 2018 19.03916 
 2019 20.96204