# year    value 
 1980 15.63427 
 1981 15.66775 
 1982 15.34836 
 1983 15.92381 
 1984 15.32577 
 1985 15.42116 
 1986 15.29239 
 1987 15.66734 
 1988 15.78347 
 1989 15.63948 
 1990 15.88657 
 1991 15.76174 
 1992 15.14129 
 1993 15.31664 
 1994 15.99841 
 1995 16.02026 
 1996 15.63723 
 1997 16.09178 
 1998 15.99683 
 1999 15.91322 
 2000 15.63328 
 2001 16.17031 
 2002 16.16311 
 2003 16.33839 
 2004 16.35483 
 2005 16.67411 
 2006 16.40179 
 2007 16.40351 
 2008 16.45779 
 2009 16.46264 
 2010 16.65254 
 2011 16.36289 
 2012 16.58254 
 2013 16.51329 
 2014 16.30002 
 2015 16.84367 
 2016  16.6258 
 2017 16.61655 
 2018 16.49343