# year    value 
 1979 7.541635 
 1980 7.595122 
 1981 7.367059 
 1982 7.412872 
 1983 8.081371 
 1984   7.1246 
 1985  6.85996 
 1986 7.000799 
 1987 7.174586 
 1988 7.843713 
 1989 7.638806 
 1990 8.205996 
 1991  7.77829 
 1992 7.149024 
 1993 6.494743 
 1994 8.316128 
 1995 8.041237 
 1996  7.13332 
 1997 7.664286 
 1998 8.058267 
 1999 8.137877 
 2000 7.265297 
 2001 8.503387 
 2002 7.781993 
 2003  8.10393 
 2004 8.533703 
 2005  8.84455 
 2006 8.455847 
 2007 8.438711 
 2008 8.527439 
 2009 8.265869 
 2010 8.702211 
 2011  8.41551 
 2012 8.382059 
 2013 8.569793 
 2014 8.016439 
 2015 8.647481 
 2016 8.218279 
 2017  8.46074 
 2018 8.064636