# year    value 
 1979 5.918743 
 1980 5.779501 
 1981 6.899821 
 1982 6.092625 
 1983 6.258235 
 1984 6.185451 
 1985 6.265324 
 1986 6.597502 
 1987 6.094816 
 1988 6.476467 
 1989 6.776361 
 1990 7.241929 
 1991 6.484834 
 1992 6.332455 
 1993 6.408329 
 1994 6.753908 
 1995 6.649011 
 1996 5.833701 
 1997 6.800345 
 1998 6.990233 
 1999 6.739298 
 2000 7.611286 
 2001 7.617027 
 2002 6.888217 
 2003 6.534503 
 2004  7.21137 
 2005 7.216308 
 2006 7.390768 
 2007  7.55638 
 2008 7.772455 
 2009 7.049395 
 2010 7.183022 
 2011  6.96029 
 2012 7.856104 
 2013 7.075471 
 2014 7.445305 
 2015 7.567007 
 2016 8.126941 
 2017 7.784303 
 2018 7.668092