# year    value 
 1980 23.33783 
 1981 22.69914 
 1982 22.01104 
 1983 23.05192 
 1984 22.92796 
 1985 22.15072 
 1986 22.65052 
 1987 22.36788 
 1988 23.57575 
 1989 22.35032 
 1990 22.83199 
 1991  22.7633 
 1992 21.03494 
 1993 21.56422 
 1994 22.99723 
 1995 22.70096 
 1996 22.89127 
 1997 22.28646 
 1998 22.84924 
 1999 22.30641 
 2000 22.83927 
 2001  23.2994 
 2002 23.20334 
 2003 23.28075 
 2004 21.47758 
 2005 22.86523 
 2006 23.82685 
 2007 23.24928 
 2008 22.71634 
 2009 22.16189 
 2010 23.09404 
 2011 23.87691 
 2012 24.04769 
 2013 23.19908 
 2014 22.65248 
 2015 23.29115 
 2016 23.50011 
 2017 22.96692 
 2018 23.62167