# year    value 
 1980 20.25408 
 1981 19.90929 
 1982 20.13544 
 1983 19.58942 
 1984 19.69763 
 1985 20.17995 
 1986 20.05036 
 1987 20.23563 
 1988 20.69937 
 1989 19.55401 
 1990 20.35882 
 1991 20.68642 
 1992 19.70924 
 1993 19.93333 
 1994 20.48433 
 1995 20.12784 
 1996 20.63703 
 1997 20.22891 
 1998 20.76807 
 1999 20.77622 
 2000 20.09163 
 2001  20.3019 
 2002 20.70214 
 2003 20.91289 
 2004 20.67598 
 2005 20.76105 
 2006  21.1235 
 2007  20.6045 
 2008 20.20454 
 2009 21.01234 
 2010 21.60571 
 2011 20.66612 
 2012 20.27901 
 2013 21.02525 
 2014 20.65556 
 2015 20.76306 
 2016 21.02342 
 2017 21.21915 
 2018 21.00718