# year    value 
 1980 20.20056 
 1981 19.98539 
 1982 20.23017 
 1983 20.29599 
 1984 20.07477 
 1985 20.07888 
 1986 20.26229 
 1987 20.58401 
 1988  20.3868 
 1989 20.03639 
 1990 20.20498 
 1991 20.46869 
 1992 20.33603 
 1993 20.46631 
 1994 20.28377 
 1995 20.73241 
 1996  20.4509 
 1997 20.61819 
 1998 21.00812 
 1999 20.48336 
 2000 20.30699 
 2001 20.56523 
 2002 20.67353 
 2003 20.64243 
 2004 20.47049 
 2005 20.93473 
 2006 20.63209 
 2007 20.27468 
 2008 20.53754 
 2009 20.75372 
 2010 20.61774 
 2011 20.53226 
 2012 20.50895 
 2013 20.72398 
 2014 20.74841 
 2015 21.06683 
 2016  20.8883 
 2017 21.06791 
 2018 20.54848