# year    value 
 1980 15.44176 
 1981 15.29279 
 1982 15.32014 
 1983 15.20006 
 1984 14.62698 
 1985 15.29866 
 1986  15.0344 
 1987 15.19167 
 1988 15.22297 
 1989 15.32587 
 1990 15.29556 
 1991 15.29695 
 1992 14.78121 
 1993 15.17894 
 1994 15.40525 
 1995 14.98544 
 1996 15.11105 
 1997 15.36293 
 1998 15.36285 
 1999 15.36429 
 2000   15.085 
 2001 15.28293 
 2002 15.23877 
 2003 15.38747 
 2004  15.4213 
 2005   15.384 
 2006 15.46645 
 2007 15.26757 
 2008 15.48061 
 2009 15.65001 
 2010 15.31576 
 2011 15.49416 
 2012 15.53919 
 2013 15.78732 
 2014 15.80162 
 2015 15.68403 
 2016 15.55662 
 2017  15.8642 
 2018  15.7798