# year    value 
 1980 18.60259 
 1981 19.12094 
 1982  18.3888 
 1983 18.32055 
 1984 18.63711 
 1985 18.35487 
 1986 18.45259 
 1987 18.46146 
 1988 18.33587 
 1989 18.47337 
 1990 18.55499 
 1991 18.27873 
 1992 18.27177 
 1993  18.3154 
 1994 18.87924 
 1995 18.64883 
 1996 18.23581 
 1997 19.30901 
 1998 19.07072 
 1999 19.01538 
 2000 19.74331 
 2001 19.71654 
 2002 19.60886 
 2003 18.60407 
 2004 19.93734 
 2005  19.4564 
 2006 19.55906 
 2007 19.61279 
 2008 20.39179 
 2009 19.76294 
 2010 19.30239 
 2011 19.01935 
 2012 19.69783 
 2013  19.9388 
 2014 19.46696 
 2015 19.52969 
 2016 20.31787 
 2017 19.91179 
 2018 20.53797