# year    value 
 1980 27.58069 
 1981 27.58785 
 1982 27.66186 
 1983 27.95902 
 1984 27.51596 
 1985 27.46706 
 1986 27.62364 
 1987 28.15749 
 1988 27.68229 
 1989 27.62982 
 1990 27.78691 
 1991 27.64796 
 1992 27.60998 
 1993 27.82511 
 1994 27.57218 
 1995 28.01549 
 1996 27.72899 
 1997 27.96346 
 1998 28.26879 
 1999 27.69479 
 2000 27.79798 
 2001 27.83911 
 2002 28.15062 
 2003 28.00956 
 2004 27.92834 
 2005 28.09792 
 2006 28.14379 
 2007 27.94387 
 2008 27.90307 
 2009 28.39997 
 2010 28.22369 
 2011 28.22145 
 2012 28.05456 
 2013 27.94793 
 2014 28.23961 
 2015 28.36471 
 2016 28.26682 
 2017 28.23534 
 2018 28.09934