# year    value 
 1980 23.59919 
 1981 23.45216 
 1982 23.13733 
 1983 23.60514 
 1984 22.96785 
 1985 23.27367 
 1986 23.12381 
 1987 23.12074 
 1988 22.95721 
 1989 22.74403 
 1990 22.98709 
 1991  23.7084 
 1992 23.60214 
 1993 23.39728 
 1994 23.65552 
 1995   23.178 
 1996 23.29111 
 1997 23.62577 
 1998 23.62992 
 1999 22.66386 
 2000 23.02594 
 2001 23.02967 
 2002 23.45486 
 2003 23.34604 
 2004 23.27959 
 2005 23.28471 
 2006 23.14785 
 2007 22.80898 
 2008 22.83039 
 2009 23.82837 
 2010 23.52313 
 2011 23.16948 
 2012 23.44144 
 2013 23.30047 
 2014 23.23384 
 2015 23.97373 
 2016 23.28794 
 2017 23.50423 
 2018 23.81908