# year    value 
 1901 7.288625 
 1902 7.117061 
 1903 7.210257 
 1904 7.140249 
 1905  7.00431 
 1906 7.580008 
 1907 7.130063 
 1908 7.573704 
 1909 7.495704 
 1910 7.334035 
 1911 7.450064 
 1912  7.09919 
 1913 7.347743 
 1914  7.48003 
 1915 7.598516 
 1916 7.174284 
 1917 7.316813 
 1918 6.943549 
 1919 7.274113 
 1920 7.475798 
 1921 7.614391 
 1922 7.357059 
 1923 7.588753 
 1924 7.328754 
 1925 7.258972 
 1926 7.480106 
 1927 7.719344 
 1928 7.461251 
 1929 7.226139 
 1930  7.79342 
 1931 7.892856 
 1932 7.836538 
 1933 7.281906 
 1934 7.448284 
 1935  7.58328 
 1936 8.085676 
 1937 7.703209 
 1938 7.875471 
 1939 7.582603 
 1940 7.627279 
 1941 7.964254 
 1942 7.525908 
 1943 7.713747 
 1944 7.327577 
 1945 7.795215 
 1946 7.370171 
 1947 7.210973 
 1948 7.728597 
 1949 7.003092 
 1950 7.160303 
 1951 7.373741 
 1952 7.460473 
 1953 8.107026 
 1954 8.119537 
 1955    7.418 
 1956 7.419744 
 1957 7.938685 
 1958 7.505225 
 1959 7.569773 
 1960 7.908581 
 1961  7.54711 
 1962 7.539193 
 1963 7.181243 
 1964 7.389465 
 1965 6.938574 
 1966 7.521826 
 1967 7.522572 
 1968 7.098854 
 1969 7.467731 
 1970 7.212451 
 1971 7.761934 
 1972 7.134465 
 1973  7.93662 
 1974 7.665349 
 1975  7.68295 
 1976 7.259346 
 1977 7.764167 
 1978 6.755191 
 1979 7.210492 
 1980 7.632801 
 1981 7.867605 
 1982 7.317289 
 1983  7.65121 
 1984   7.7888 
 1985 7.477737 
 1986 7.206899 
 1987 7.279262 
 1988 8.119273 
 1989  7.97977 
 1990 8.312925 
 1991 8.424637 
 1992  7.04037 
 1993 7.917585 
 1994 8.097969 
 1995 8.167635 
 1996 7.546225 
 1997 8.028769 
 1998 8.635311 
 1999  7.75918 
 2000 8.078775 
 2001 8.506304 
 2002 8.431825 
 2003 8.592057 
 2004 8.018384 
 2005 8.454182 
 2006 8.452615 
 2007 8.664599 
 2008 8.249965 
 2009 8.219307 
 2010 8.536912 
 2011 8.614388 
 2012 8.648458 
 2013 8.727873 
 2014 8.538341 
 2015 8.333937 
 2016 9.139276 
 2017 8.470943 
 2018 8.364129