# year    value 
 1979 20.06593 
 1980 19.05177 
 1981 19.28308 
 1982 19.12407 
 1983 19.37865 
 1984 19.09495 
 1985 18.87427 
 1986 18.72258 
 1987 18.96792 
 1988 18.89376 
 1989 19.37688 
 1990 19.26349 
 1991 19.18162 
 1992   18.377 
 1993 18.76084 
 1994 18.64361 
 1995 19.21469 
 1996 18.67903 
 1997 18.99957 
 1998 18.85579 
 1999 19.39628 
 2000 18.82952 
 2001  19.5319 
 2002 18.57587 
 2003 19.26351 
 2004 19.26902 
 2005 19.18781 
 2006   19.319 
 2007 19.27788 
 2008 19.34052 
 2009 19.57835 
 2010 19.27519 
 2011 19.22003 
 2012 19.37101 
 2013 19.64679 
 2014 19.68934 
 2015 19.36633 
 2016 19.76195 
 2017  19.7224 
 2018 19.96202