# year    value 
 1979 19.99838 
 1980 19.52846 
 1981 20.35437 
 1982 19.34675 
 1983 19.06203 
 1984 19.41655 
 1985  20.4427 
 1986 20.13979 
 1987 19.53646 
 1988 20.53335 
 1989 19.89919 
 1990 20.06793 
 1991 19.54402 
 1992 19.60956 
 1993 18.20117 
 1994 21.48065 
 1995 19.37907 
 1996 20.92056 
 1997 19.59329 
 1998 20.00313 
 1999  19.3263 
 2000  20.7362 
 2001 20.53819 
 2002 21.10091 
 2003 21.59007 
 2004 20.31033 
 2005 20.35217 
 2006 21.45279 
 2007 21.55794 
 2008 20.65418 
 2009 20.17505 
 2010 20.23981 
 2011 20.45298 
 2012 21.45477 
 2013 21.44575 
 2014 20.59892 
 2015 21.41635 
 2016 21.40027 
 2017 21.88591 
 2018 21.65617