# year    value 
 1980  18.8807 
 1981 19.17745 
 1982 18.65746 
 1983 19.01533 
 1984 18.90225 
 1985 18.71048 
 1986 18.86398 
 1987 18.84006 
 1988 19.46128 
 1989 19.13373 
 1990 19.17413 
 1991 19.35259 
 1992 18.34811 
 1993 18.54895 
 1994 19.42432 
 1995 19.43964 
 1996 19.13032 
 1997 19.19966 
 1998 19.81187 
 1999 19.56771 
 2000 19.53619 
 2001 19.64945 
 2002 19.61528 
 2003 19.52119 
 2004 19.11776 
 2005 19.69601 
 2006 19.89723 
 2007 19.84036 
 2008 19.74752 
 2009 19.23943 
 2010 20.16747 
 2011 19.88844 
 2012 20.22903 
 2013 19.74444 
 2014   19.626 
 2015 19.92268 
 2016 20.14721 
 2017 19.95103 
 2018 20.15748