# year    value 
 1979 22.43878 
 1980 22.60818 
 1981 22.39085 
 1982 22.52079 
 1983 22.64152 
 1984 22.02832 
 1985 22.24456 
 1986 22.34503 
 1987  22.6553 
 1988 22.39792 
 1989 22.44379 
 1990 22.38053 
 1991  22.4522 
 1992 22.26082 
 1993 22.30889 
 1994 22.66179 
 1995 22.68197 
 1996 22.37927 
 1997  23.0128 
 1998 22.84199 
 1999 22.28457 
 2000 22.18935 
 2001 22.52759 
 2002 22.74972 
 2003 22.78719 
 2004  22.6941 
 2005 22.81757 
 2006 22.96397 
 2007  22.5501 
 2008 22.83581 
 2009  23.0646 
 2010 23.01065 
 2011 22.89418 
 2012 23.01035 
 2013 22.88401 
 2014 22.90828 
 2015 23.31111 
 2016 23.07243 
 2017  23.1088 
 2018 22.78831