# year    value 
 1901 6.920291 
 1902  6.60754 
 1903 6.753079 
 1904 7.175288 
 1905 7.394098 
 1906 6.910677 
 1907 6.662322 
 1908 6.803369 
 1909 7.512147 
 1910 6.781901 
 1911  6.93667 
 1912 6.146463 
 1913 7.319817 
 1914 6.907688 
 1915 7.022362 
 1916 6.818167 
 1917  7.27347 
 1918 7.167713 
 1919 6.795716 
 1920 6.673353 
 1921 6.941522 
 1922 7.084431 
 1923  7.64917 
 1924   7.3778 
 1925 7.286178 
 1926 7.260475 
 1927 7.739711 
 1928  7.27086 
 1929 7.280689 
 1930 7.342857 
 1931 7.660077 
 1932 7.488462 
 1933 7.296586 
 1934 7.378529 
 1935 7.034387 
 1936 7.381974 
 1937 7.508859 
 1938 7.901205 
 1939 7.221169 
 1940 7.369315 
 1941  6.99354 
 1942 7.521516 
 1943 7.430127 
 1944 7.340386 
 1945  7.47186 
 1946  7.26296 
 1947 7.694949 
 1948 7.533968 
 1949 7.347009 
 1950 6.778128 
 1951 7.104818 
 1952 6.827344 
 1953 7.312222 
 1954 7.817704 
 1955 7.392456 
 1956 6.900687 
 1957 7.311854 
 1958 7.337309 
 1959  6.66203 
 1960 7.204953 
 1961  7.22651 
 1962 7.328987 
 1963 8.197292 
 1964 7.081488 
 1965 7.016181 
 1966 7.192529 
 1967  7.40645 
 1968 7.088244 
 1969 7.160604 
 1970 7.171717 
 1971 7.795355 
 1972 6.637755 
 1973   7.3076 
 1974 7.092326 
 1975 7.439648 
 1976 6.246938 
 1977 7.652507 
 1978   7.3989 
 1979 7.465554 
 1980 7.571723 
 1981 7.306008 
 1982 7.358559 
 1983 8.022329 
 1984 7.000632 
 1985  6.83294 
 1986 6.967304 
 1987 7.100062 
 1988 7.756977 
 1989 7.557698 
 1990 8.099083 
 1991   7.6498 
 1992 7.045238 
 1993 6.473069 
 1994  8.18156 
 1995 7.928555 
 1996 7.012037 
 1997 7.507891 
 1998  7.93812 
 1999 8.065357 
 2000 7.145411 
 2001 8.333234 
 2002 7.665205 
 2003 7.928533 
 2004 8.349347 
 2005 8.683784 
 2006 8.240226 
 2007 8.189932 
 2008 8.344511 
 2009 8.055293 
 2010 8.508097 
 2011 8.159076 
 2012 8.183402 
 2013 8.407548 
 2014   7.9259 
 2015 8.389146 
 2016 8.005039 
 2017 8.290092 
 2018 7.963048