# year    value 
 1960 6.168694 
 1961 6.603249 
 1962 6.466275 
 1963 6.512894 
 1964 5.956351 
 1965 6.018569 
 1966 6.261423 
 1967 6.215574 
 1968 6.160894 
 1969 5.636863 
 1970 6.156303 
 1971 6.356402 
 1972 5.719525 
 1973 6.546055 
 1974 6.081578 
 1975 6.569221 
 1976 5.856501 
 1977 6.491342 
 1978 6.236472 
 1979  6.37472 
 1980 6.278632 
 1981 6.932093 
 1982  6.32656 
 1983  6.80041 
 1984 6.143197 
 1985 6.049151 
 1986 6.524944 
 1987 6.566045 
 1988 6.819425 
 1989 6.830188 
 1990 7.168651 
 1991 6.783163 
 1992 6.518674 
 1993 6.316416 
 1994 6.888136 
 1995 7.129692 
 1996 6.275736 
 1997 6.859851 
 1998 7.361715 
 1999 7.431127 
 2000 7.024742 
 2001 7.292842 
 2002 7.264273 
 2003 7.038678 
 2004 7.222356 
 2005 7.212891 
 2006 7.445947 
 2007 7.574523 
 2008 7.257182 
 2009 6.992707 
 2010 7.286646 
 2011 7.097534 
 2012 7.258273 
 2013 7.269231 
 2014 7.189088 
 2015 7.775247 
 2016 7.750423 
 2017 7.698007 
 2018 7.396747