# year    value 
 1960 8.226181 
 1961  8.27139 
 1962 8.329765 
 1963 8.451289 
 1964  7.85643 
 1965 7.990999 
 1966 8.133275 
 1967 8.114163 
 1968 8.061135 
 1969 8.068796 
 1970  8.19087 
 1971 8.073589 
 1972 7.963092 
 1973   8.3562 
 1974 7.940152 
 1975 8.199293 
 1976 7.936185 
 1977 8.321955 
 1978 8.392752 
 1979 8.353729 
 1980 8.447549 
 1981 8.631359 
 1982  8.22064 
 1983 8.533908 
 1984 8.124569 
 1985  8.22398 
 1986 8.473589 
 1987 8.517636 
 1988 8.733828 
 1989 8.584625 
 1990 8.800265 
 1991 8.612941 
 1992 8.407313 
 1993 8.513351 
 1994  8.78204 
 1995  8.94179 
 1996 8.471911 
 1997 8.817871 
 1998 9.105676 
 1999 8.849308 
 2000 8.725664 
 2001 8.944947 
 2002 9.097575 
 2003 9.076939 
 2004 8.989868 
 2005 9.038742 
 2006 9.068454 
 2007 9.238015 
 2008 8.961401 
 2009 9.011814 
 2010 9.124215 
 2011 8.951745 
 2012 8.900222 
 2013 9.091536 
 2014 9.242757 
 2015 9.509169 
 2016 9.536229 
 2017 9.470224 
 2018 9.338805