# year    value 
 1979 6.153407 
 1980 6.998628 
 1981 7.201794 
 1982 6.750719 
 1983 7.137373 
 1984 6.605176 
 1985 6.207016 
 1986 6.861126 
 1987  6.93585 
 1988 7.168005 
 1989 6.946441 
 1990 7.047937 
 1991  7.12356 
 1992 7.028721 
 1993 6.983968 
 1994 6.697271 
 1995 7.519264 
 1996 6.945915 
 1997 7.045667 
 1998 7.426218 
 1999 7.589655 
 2000 7.284417 
 2001 6.922612 
 2002 7.667606 
 2003 7.338289 
 2004 7.610881 
 2005 7.303299 
 2006 7.502257 
 2007 8.026381 
 2008 7.135208 
 2009  7.46192 
 2010 7.522048 
 2011 7.141722 
 2012 7.222858 
 2013 7.360127 
 2014 7.358374 
 2015 7.857213 
 2016 8.499951 
 2017 8.046794 
 2018 7.877107