# year    value 
 1979 28.39191 
 1980 28.35308 
 1981 28.32368 
 1982 28.26973 
 1983 28.63443 
 1984 27.98948 
 1985 28.00121 
 1986 28.27361 
 1987 28.89594 
 1988 28.47352 
 1989 28.36631 
 1990 28.53891 
 1991  28.4547 
 1992 28.30405 
 1993 28.55719 
 1994 28.31048 
 1995 28.78971 
 1996 28.41309 
 1997 28.74585 
 1998 29.07566 
 1999 28.46874 
 2000 28.45239 
 2001 28.58887 
 2002  28.9074 
 2003 28.72088 
 2004 28.64558 
 2005 28.90486 
 2006 28.93498 
 2007 28.77266 
 2008 28.72116 
 2009 29.20095 
 2010 29.17614 
 2011 29.04386 
 2012 28.94536 
 2013 28.74704 
 2014 29.17703 
 2015 29.32687 
 2016 29.20594 
 2017 29.24373 
 2018 29.11024