# year    value 
 1980 14.49337 
 1981 15.15228 
 1982  16.1706 
 1983  16.6863 
 1984 14.75751 
 1985 15.41303 
 1986 15.44514 
 1987 15.20809 
 1988 15.85138 
 1989 15.46126 
 1990 15.85057 
 1991 16.23966 
 1992   16.673 
 1993 15.86575 
 1994 17.42341 
 1995 15.98267 
 1996 15.53719 
 1997 15.75978 
 1998 16.68629 
 1999 16.16698 
 2000 16.41751 
 2001 16.32813 
 2002 16.68825 
 2003  18.4436 
 2004 16.11892 
 2005 16.10372 
 2006 16.62099 
 2007 16.48314 
 2008 16.57529 
 2009  16.8849 
 2010 16.76393 
 2011 16.35146 
 2012 17.51866 
 2013 17.10641 
 2014 15.98499 
 2015  18.3694 
 2016 16.90186 
 2017 18.10383 
 2018 18.07128