# year    value 
 1980 18.79968 
 1981 19.57735 
 1982 18.69666 
 1983 18.62027 
 1984 18.94998 
 1985 19.58977 
 1986 19.53678 
 1987 18.80921 
 1988 19.76094 
 1989 19.11463 
 1990  19.4176 
 1991 18.95578 
 1992 18.96406 
 1993 17.77357 
 1994 20.46542 
 1995  18.7903 
 1996 20.12112 
 1997 18.83188 
 1998 19.17825 
 1999 18.56801 
 2000 19.61877 
 2001 19.81246 
 2002 19.81073 
 2003 20.47215 
 2004 19.40957 
 2005  19.4202 
 2006 20.47135 
 2007 20.39806 
 2008 19.62354 
 2009 19.32842 
 2010 19.26997 
 2011 19.48844 
 2012 20.30105 
 2013 20.49101 
 2014 19.97706 
 2015 20.61117 
 2016 20.29483 
 2017 20.17067 
 2018 20.51968