# year    value 
 1980 8.511888 
 1981 8.674057 
 1982 8.229405 
 1983 8.539632 
 1984  8.18416 
 1985 8.236253 
 1986 8.460411 
 1987  8.60075 
 1988 8.844792 
 1989 8.615312 
 1990 8.859426 
 1991 8.689627 
 1992 8.406854 
 1993 8.605202 
 1994 8.899123 
 1995 8.988495 
 1996 8.494356 
 1997 8.794372 
 1998 9.184965 
 1999 8.901727 
 2000 8.745688 
 2001 9.017402 
 2002 9.161126 
 2003 9.180011 
 2004 9.071955 
 2005 9.192895 
 2006 9.211134 
 2007 9.401077 
 2008 9.102375 
 2009  9.14548 
 2010 9.337336 
 2011 9.078684 
 2012 9.072972 
 2013 9.282067 
 2014 9.428958 
 2015 9.729979 
 2016 9.850148 
 2017 9.734256 
 2018 9.568665