# year    value 
 1980 15.57573 
 1981 15.58765 
 1982 15.31386 
 1983 15.85351 
 1984 15.25527 
 1985 15.37366 
 1986 15.27001 
 1987 15.57315 
 1988  15.6673 
 1989  15.5137 
 1990 15.78519 
 1991 15.72639 
 1992 15.09483 
 1993 15.22411 
 1994  15.9157 
 1995 15.94068 
 1996 15.52288 
 1997 15.97653 
 1998 15.90481 
 1999 15.88766 
 2000 15.59235 
 2001 16.06895 
 2002 16.02638 
 2003  16.1677 
 2004 16.23309 
 2005 16.53404 
 2006 16.17313 
 2007 16.25736 
 2008 16.27468 
 2009 16.27868 
 2010 16.42897 
 2011 16.17194 
 2012 16.32482 
 2013 16.29597 
 2014 16.11386 
 2015 16.50795 
 2016 16.27103 
 2017 16.31446 
 2018 16.17954