# year    value 
 1979  19.1447 
 1980 19.27073 
 1981 19.09244 
 1982 19.24075 
 1983 19.35996 
 1984 18.69554 
 1985 18.93236 
 1986 19.03814 
 1987 19.31999 
 1988 19.12472 
 1989 19.11395 
 1990  19.1141 
 1991 19.19498 
 1992 18.96201 
 1993 19.04929 
 1994 19.43592 
 1995 19.45906 
 1996 19.10063 
 1997 19.70516 
 1998  19.6517 
 1999 19.00414 
 2000 18.93661 
 2001 19.25649 
 2002 19.43853 
 2003 19.56792 
 2004 19.42836 
 2005 19.49981 
 2006 19.71249 
 2007 19.26532 
 2008 19.47489 
 2009  19.7477 
 2010 19.71445 
 2011  19.5366 
 2012 19.72635 
 2013 19.57379 
 2014 19.64457 
 2015 20.01245 
 2016 19.85323 
 2017 19.81773 
 2018  19.5326