# year    value 
 1980  18.2149 
 1981 18.79184 
 1982 18.08398 
 1983 17.96268 
 1984 18.31039 
 1985 18.08704 
 1986 18.12497 
 1987 18.09744 
 1988 18.10376 
 1989 18.17758 
 1990  18.3242 
 1991 18.03176 
 1992 18.05262 
 1993 18.06049 
 1994 18.63675 
 1995 18.23674 
 1996  17.9862 
 1997 18.91321 
 1998 18.87912 
 1999 18.72822 
 2000 19.33619 
 2001 19.35446 
 2002 19.38707 
 2003 18.33724 
 2004 19.65654 
 2005 19.02279 
 2006 19.25207 
 2007  19.3901 
 2008 20.02915 
 2009 19.44494 
 2010 19.03039 
 2011 18.84861 
 2012  19.1756 
 2013 19.43564 
 2014 18.83627 
 2015 19.14365 
 2016 19.86441 
 2017 19.49349 
 2018 20.09915