# year    value 
 1979 20.39676 
 1980 20.51708 
 1981 20.26681 
 1982 20.56408 
 1983 20.60517 
 1984 20.32337 
 1985 20.19895 
 1986 20.39796 
 1987 20.81362 
 1988 20.67488 
 1989 20.31895 
 1990 20.52354 
 1991 20.67787 
 1992 20.44917 
 1993 20.60473 
 1994 20.40966 
 1995 21.00749 
 1996 20.68445 
 1997 20.83706 
 1998 21.23747 
 1999  20.7021 
 2000 20.42872 
 2001 20.73353 
 2002  20.9081 
 2003 20.85925 
 2004  20.6476 
 2005 21.19474 
 2006 20.91928 
 2007 20.61615 
 2008 20.80371 
 2009 21.01586 
 2010 21.01025 
 2011 20.76689 
 2012 20.80793 
 2013 21.09186 
 2014  21.0848 
 2015 21.45475 
 2016 21.32393 
 2017 21.43863 
 2018 20.98329