# year    value 
 1980 4.375147 
 1981 3.539768 
 1982  3.71093 
 1983 4.646242 
 1984 3.614978 
 1985 3.291756 
 1986 3.757249 
 1987 4.035888 
 1988 4.824793 
 1989 4.337425 
 1990 4.978612 
 1991 4.442602 
 1992 3.879967 
 1993 3.872303 
 1994 4.712426 
 1995 4.923832 
 1996  3.62952 
 1997 4.126771 
 1998 4.880658 
 1999 4.756574 
 2000 3.735571 
 2001 5.220312 
 2002 4.370736 
 2003 4.633886 
 2004 4.887447 
 2005 5.129524 
 2006 5.053473 
 2007 5.099669 
 2008 5.100551 
 2009 4.573246 
 2010 5.255649 
 2011 4.964716 
 2012 4.938492 
 2013 5.075348 
 2014 5.088817 
 2015  5.29599 
 2016 4.846845 
 2017 5.090703 
 2018 4.882521