# year    value 
 1979  19.5085 
 1980 18.83909 
 1981  19.4763 
 1982 20.46858 
 1983 20.97808 
 1984 18.96314 
 1985 19.63021 
 1986  19.7442 
 1987 19.65536 
 1988 20.17952 
 1989 19.82464 
 1990 19.99278 
 1991 20.27775 
 1992 20.98689 
 1993 20.07549 
 1994 21.60124 
 1995 20.30061 
 1996 19.76612 
 1997 20.16125 
 1998 20.88015 
 1999 20.44209 
 2000 20.73323 
 2001 20.64537 
 2002 20.87979 
 2003 23.63351 
 2004 20.45642 
 2005 20.30006 
 2006 20.92149 
 2007 20.90046 
 2008 20.82497 
 2009 21.12145 
 2010  20.9273 
 2011 20.64697 
 2012 21.87351 
 2013 21.17784 
 2014 20.28982 
 2015  22.4504 
 2016 21.02445 
 2017 22.30719 
 2018 22.24613