# year    value 
 1979 6.884953 
 1980 7.327945 
 1981 7.535959 
 1982 7.055163 
 1983 7.386015 
 1984 7.622114 
 1985 7.242404 
 1986 6.921754 
 1987 7.049753 
 1988 7.800933 
 1989 7.737177 
 1990 7.981927 
 1991 8.132051 
 1992 6.588681 
 1993 7.672863 
 1994 7.807147 
 1995 7.836994 
 1996 6.993489 
 1997 7.553564 
 1998 8.234603 
 1999 7.316353 
 2000 7.673694 
 2001 8.135026 
 2002  8.04005 
 2003 8.166795 
 2004 7.654033 
 2005 8.087385 
 2006  8.20546 
 2007 8.430676 
 2008 7.930551 
 2009 7.921527 
 2010 8.457694 
 2011 8.440419 
 2012 8.898098 
 2013 8.516788 
 2014 8.278015 
 2015  8.09418 
 2016 9.227387 
 2017  8.28268 
 2018 8.337723