# year    value 
 1979 15.83779 
 1980 15.95539 
 1981 15.61038 
 1982 16.16212 
 1983 16.15418 
 1984 15.77832 
 1985 15.68391 
 1986  15.8847 
 1987 16.27403 
 1988 16.09116 
 1989  15.9704 
 1990 15.99589 
 1991 16.03938 
 1992 15.78947 
 1993 16.10783 
 1994 16.10266 
 1995 16.62411 
 1996 15.92063 
 1997 16.44069 
 1998  16.7269 
 1999 16.07983 
 2000 15.98023 
 2001 16.24271 
 2002 16.30228 
 2003 16.28585 
 2004 16.14008 
 2005 16.67132 
 2006 16.49606 
 2007 16.16106 
 2008 16.30334 
 2009 16.39342 
 2010 16.51814 
 2011 16.26341 
 2012 16.52758 
 2013 16.44192 
 2014  16.7284 
 2015 17.03837 
 2016 16.79111 
 2017  16.8495 
 2018 16.50598