# year    value 
 1980 23.67433 
 1981 23.38256 
 1982 23.13323 
 1983 23.82066 
 1984 23.29037 
 1985 23.40032 
 1986 23.64849 
 1987 23.72998 
 1988  23.6682 
 1989 23.21776 
 1990 23.65993 
 1991 23.62486 
 1992 23.89692 
 1993 23.72872 
 1994 23.53414 
 1995 23.46016 
 1996 23.41399 
 1997 23.30272 
 1998 24.15842 
 1999 23.45323 
 2000 23.28154 
 2001 23.53132 
 2002 24.06928 
 2003 23.79934 
 2004 23.71252 
 2005 24.13075 
 2006 23.38082 
 2007 23.77608 
 2008 23.25321 
 2009 23.69153 
 2010 24.00763 
 2011 23.33664 
 2012 23.44948 
 2013 23.75227 
 2014 23.83089 
 2015 23.78902 
 2016 24.18429 
 2017 23.91039 
 2018 23.95703