# year    value 
 1979 17.65375 
 1980 17.13848 
 1981 17.78867 
 1982 18.72602 
 1983 19.18439 
 1984 17.22538 
 1985 17.95605 
 1986 18.00802 
 1987 17.96169 
 1988 18.43974 
 1989 18.00925 
 1990 18.21665 
 1991 18.60629 
 1992 19.14462 
 1993 18.34419 
 1994 19.89175 
 1995 18.47753 
 1996 18.05525 
 1997 18.35803 
 1998 19.23756 
 1999 18.70202 
 2000 18.96755 
 2001 18.93764 
 2002 19.17788 
 2003 21.85175 
 2004 18.75449 
 2005 18.61586 
 2006 19.16102 
 2007 19.16511 
 2008 19.16564 
 2009 19.45307 
 2010 19.30753 
 2011 18.96125 
 2012 20.21579 
 2013 19.50101 
 2014 18.49827 
 2015 20.82116 
 2016 19.36998 
 2017 20.62891 
 2018 20.44618