# year    value 
 1979 18.93612 
 1980 19.66311 
 1981 19.59502 
 1982 19.23091 
 1983  19.9554 
 1984 18.88163 
 1985 19.20491 
 1986 19.07544 
 1987  19.0888 
 1988 19.13305 
 1989 18.75227 
 1990 19.19805 
 1991 19.64745 
 1992 19.50064 
 1993 19.25378 
 1994 19.48053 
 1995 19.31246 
 1996 19.31184 
 1997 19.49546 
 1998 19.73011 
 1999 18.76834 
 2000 18.95671 
 2001 19.08836 
 2002 19.58878 
 2003 19.41972 
 2004 19.20984 
 2005 19.38569 
 2006 19.21471 
 2007 19.13087 
 2008 19.07508 
 2009 20.02318 
 2010 19.85561 
 2011 19.39387 
 2012 19.60537 
 2013  19.5705 
 2014 19.29777 
 2015 20.02461 
 2016 19.65004 
 2017 19.61116 
 2018 19.85925