# year    value 
 1980 3.893517 
 1981 4.218308 
 1982  3.78981 
 1983  4.01676 
 1984 3.523589 
 1985 3.618359 
 1986 4.053373 
 1987 4.034452 
 1988 4.339038 
 1989 4.315515 
 1990 4.584019 
 1991 4.106802 
 1992 4.041449 
 1993 3.918244 
 1994 4.509593 
 1995 4.551705 
 1996 3.996416 
 1997 4.396903 
 1998 4.783379 
 1999 4.736722 
 2000 4.370972 
 2001 4.671387 
 2002 4.744669 
 2003 4.602963 
 2004 4.764667 
 2005 4.486715 
 2006 4.781437 
 2007 5.067226 
 2008 4.712748 
 2009 4.539443 
 2010  4.65297 
 2011 4.488928 
 2012 4.419275 
 2013 4.765692 
 2014 4.867874 
 2015 5.267933 
 2016 5.091512 
 2017 5.168514 
 2018 5.070175