# year    value 
 1979 6.476756 
 1980 6.414499 
 1981 7.041434 
 1982 6.429245 
 1983 6.920093 
 1984 6.276086 
 1985 6.185606 
 1986 6.612621 
 1987 6.700889 
 1988 6.950294 
 1989 6.955944 
 1990 7.303956 
 1991 6.914765 
 1992 6.629059 
 1993 6.457136 
 1994 7.031451 
 1995  7.27959 
 1996 6.420099 
 1997 7.029253 
 1998  7.50437 
 1999 7.557919 
 2000 7.189955 
 2001  7.49302 
 2002 7.438217 
 2003 7.247838 
 2004 7.401704 
 2005 7.457621 
 2006 7.662837 
 2007 7.819162 
 2008 7.503787 
 2009 7.217939 
 2010 7.581192 
 2011 7.344235 
 2012  7.48057 
 2013 7.500658 
 2014 7.409337 
 2015 8.017964 
 2016 8.010192 
 2017 7.961703 
 2018 7.603103