# year    value 
 1979 17.09728 
 1980 17.45749 
 1981 15.96561 
 1982 16.94215 
 1983 17.04096 
 1984 16.55259 
 1985 16.31384 
 1986 16.50692 
 1987 16.69361 
 1988 17.27574 
 1989 16.96408 
 1990 17.45559 
 1991 16.98738 
 1992     16.2 
 1993 16.28104 
 1994 17.02907 
 1995  17.4496 
 1996 16.76094 
 1997 16.96578 
 1998 18.05352 
 1999 17.50989 
 2000 16.67632 
 2001 17.70549 
 2002 17.32621 
 2003  17.2129 
 2004 17.39325 
 2005 17.83263 
 2006 18.29874 
 2007 17.45124 
 2008 17.73802 
 2009 17.27367 
 2010 17.84236 
 2011 17.49828 
 2012 16.90364 
 2013 17.72228 
 2014 17.50829 
 2015 17.68239 
 2016 17.27956 
 2017 17.81672 
 2018 17.13751