# year    value 
 1979 19.94392 
 1980 20.14452 
 1981 19.97479 
 1982 19.87455 
 1983 20.14503 
 1984 19.72996 
 1985 19.84269 
 1986 19.79487 
 1987  20.4857 
 1988 20.12873 
 1989  20.1815 
 1990 20.43161 
 1991 19.97117 
 1992 19.71503 
 1993  20.2749 
 1994  20.2515 
 1995 20.41714 
 1996 20.26549 
 1997 20.28601 
 1998 20.91264 
 1999 20.40692 
 2000  20.1558 
 2001 20.53382 
 2002 20.46498 
 2003 20.66711 
 2004 20.32331 
 2005 20.78718 
 2006 20.68585 
 2007 20.44763 
 2008 20.54367 
 2009  20.9318 
 2010 20.78385 
 2011 20.59579 
 2012 20.72925 
 2013 20.56778 
 2014 20.77295 
 2015 21.32033 
 2016 21.24942 
 2017 21.15993 
 2018 20.77834