# year    value 
 1980 3.556933 
 1981 3.841531 
 1982 3.350116 
 1983 3.579175 
 1984 3.206052 
 1985 3.238344 
 1986 3.664297 
 1987 3.752203 
 1988 4.030297 
 1989 3.911095 
 1990 4.244784 
 1991 3.799499 
 1992 3.696213 
 1993 3.672127 
 1994  4.25489 
 1995  4.17168 
 1996 3.590186 
 1997 4.006002 
 1998 4.447846 
 1999 4.372689 
 2000 4.049989 
 2001 4.421061 
 2002 4.489198 
 2003 4.366054 
 2004 4.484823 
 2005 4.266403 
 2006 4.543444 
 2007 4.867873 
 2008 4.483538 
 2009 4.247543 
 2010 4.481968 
 2011 4.288622 
 2012 4.196915 
 2013 4.545272 
 2014 4.681278 
 2015 5.136662 
 2016 5.017023 
 2017 5.065228 
 2018 4.877212