# year    value 
 1979 19.51852 
 1980 19.64561 
 1981 19.67734 
 1982 19.48695 
 1983 19.75369 
 1984 19.49471 
 1985 19.42531 
 1986 19.50685 
 1987 19.78601 
 1988 19.95079 
 1989 19.70993 
 1990 19.86996 
 1991 19.93274 
 1992 19.28896 
 1993 19.66286 
 1994 19.85094 
 1995 20.08345 
 1996 19.71087 
 1997 19.94881 
 1998 20.45975 
 1999 19.94325 
 2000 19.89071 
 2001 20.13093 
 2002 20.21451 
 2003 20.15461 
 2004 19.86773 
 2005 20.32482 
 2006 20.36369 
 2007 20.21288 
 2008 20.17908 
 2009 20.19607 
 2010 20.56687 
 2011  20.3709 
 2012  20.5287 
 2013  20.3483 
 2014 20.40138 
 2015 20.57922 
 2016 20.74761 
 2017 20.62661 
 2018 20.51158