# year    value 
 1980 23.25384 
 1981 23.01232 
 1982 23.23215 
 1983 23.30969 
 1984 22.90222 
 1985 23.19921 
 1986 23.24269 
 1987 23.52639 
 1988 23.24073 
 1989 23.17983 
 1990 23.17144 
 1991 23.33536 
 1992 23.22632 
 1993 23.21461 
 1994  23.5912 
 1995 23.46278 
 1996 23.17861 
 1997 23.66198 
 1998 23.59657 
 1999  22.9842 
 2000 23.08875 
 2001 23.30739 
 2002 23.40793 
 2003 23.42965 
 2004 23.49982 
 2005 23.37554 
 2006 23.53077 
 2007 23.04544 
 2008  23.2978 
 2009 23.58581 
 2010  23.4805 
 2011 23.42507 
 2012 23.61374 
 2013 23.47103 
 2014 23.70729 
 2015 23.99973 
 2016 23.62736 
 2017 23.74026 
 2018 23.44445