# year    value 
 1980 18.73396 
 1981 18.46721 
 1982 18.29703 
 1983 19.40607 
 1984 18.50795 
 1985 18.38844 
 1986 18.61055 
 1987 18.86771 
 1988 18.87683 
 1989 18.30944 
 1990 18.87415 
 1991  18.8613 
 1992 19.01205 
 1993 18.82961 
 1994 18.83905 
 1995 18.85608 
 1996  18.7485 
 1997 18.36086 
 1998 19.62502 
 1999 18.56227 
 2000 18.53156 
 2001 18.69149 
 2002 19.09717 
 2003 19.04361 
 2004 18.77282 
 2005 19.15432 
 2006 18.64244 
 2007 19.08122 
 2008  18.3921 
 2009 18.92681 
 2010 19.44529 
 2011 18.82104 
 2012  18.8996 
 2013 19.05388 
 2014 19.03338 
 2015 19.28485 
 2016 19.72833 
 2017 19.23852 
 2018 19.16075