# year    value 
 1980 6.457575 
 1981 6.743336 
 1982 6.238095 
 1983 6.679173 
 1984 6.155585 
 1985 5.700898 
 1986 6.428068 
 1987 6.485773 
 1988 6.635434 
 1989  6.47876 
 1990 6.532582 
 1991 6.599517 
 1992 6.557526 
 1993 6.529917 
 1994 6.170564 
 1995 6.999128 
 1996 6.408893 
 1997 6.563912 
 1998 6.975681 
 1999 7.155775 
 2000 6.877654 
 2001 6.506906 
 2002 7.182815 
 2003 6.806849 
 2004 7.156206 
 2005 6.705616 
 2006 6.974257 
 2007 7.424023 
 2008 6.550348 
 2009 6.864415 
 2010 6.810998 
 2011 6.554956 
 2012 6.647638 
 2013 6.741651 
 2014 6.806244 
 2015 7.327003 
 2016 7.875218 
 2017 7.398457 
 2018 7.159111