# year    value 
 1979 23.41539 
 1980 23.56437 
 1981 23.40971 
 1982 23.34579 
 1983 23.65944 
 1984 23.07159 
 1985 22.97953 
 1986 23.16427 
 1987 23.68681 
 1988 23.44939 
 1989 23.36478 
 1990 23.54563 
 1991  23.4325 
 1992 23.37602 
 1993 23.55289 
 1994 23.47131 
 1995 23.70477 
 1996 23.35365 
 1997 23.61512 
 1998 24.06041 
 1999 23.44823 
 2000  23.4413 
 2001 23.56562 
 2002 23.79837 
 2003 23.71636 
 2004  23.5802 
 2005 23.95783 
 2006 23.91098 
 2007 23.78265 
 2008 23.66526 
 2009 24.23715 
 2010 24.09722 
 2011 24.07112 
 2012 23.96057 
 2013 23.92721 
 2014 24.15137 
 2015 24.22014 
 2016 24.21168 
 2017 24.26821 
 2018 24.13592