# year    value 
 1980 5.742509 
 1981 6.935093 
 1982 6.135465 
 1983 6.263051 
 1984 6.103519 
 1985 6.264677 
 1986 6.646356 
 1987 6.075496 
 1988 6.483475 
 1989 6.789474 
 1990 7.274109 
 1991 6.490553 
 1992 6.379988 
 1993 6.430409 
 1994 6.808729 
 1995 6.667002 
 1996 5.823996 
 1997 6.772082 
 1998 7.057596 
 1999 6.737367 
 2000 7.584797 
 2001 7.528393 
 2002 6.845772 
 2003 6.510767 
 2004 7.177596 
 2005 7.084811 
 2006 7.339467 
 2007   7.5045 
 2008 7.689529 
 2009 7.013508 
 2010 7.089259 
 2011 6.938289 
 2012 7.792279 
 2013 6.931713 
 2014 7.282174 
 2015 7.486171 
 2016 7.994559 
 2017  7.70393 
 2018 7.554878