# year    value 
 1979 23.73104 
 1980  23.7886 
 1981  23.5679 
 1982 23.63732 
 1983 23.85282 
 1984 23.57655 
 1985 23.54462 
 1986 23.47355 
 1987 24.15525 
 1988 23.99457 
 1989 23.75425 
 1990 24.01667 
 1991 23.78898 
 1992 23.49433 
 1993 23.88878 
 1994 24.06219 
 1995 24.17475 
 1996 23.91389 
 1997 24.44214 
 1998 24.20814 
 1999 23.71238 
 2000 23.83994 
 2001 23.88886 
 2002 24.34098 
 2003 24.24576 
 2004 24.22774 
 2005 24.33362 
 2006 24.26797 
 2007 24.36692 
 2008 24.40076 
 2009 24.52864 
 2010 24.19257 
 2011 24.07226 
 2012 24.59795 
 2013  24.4655 
 2014 24.64489 
 2015 25.02463 
 2016 24.50162 
 2017 24.58065 
 2018 24.62529