# year    value 
 1901 7.160513 
 1902  7.00633 
 1903  7.09569 
 1904 7.096465 
 1905 6.924335 
 1906 7.489808 
 1907 7.056746 
 1908 7.447157 
 1909 7.402639 
 1910 7.169631 
 1911  7.34073 
 1912 7.032729 
 1913 7.260928 
 1914 7.296448 
 1915 7.503638 
 1916 7.062572 
 1917  7.21664 
 1918  6.80487 
 1919 7.214044 
 1920 7.416997 
 1921 7.499179 
 1922 7.296376 
 1923 7.463591 
 1924 7.250499 
 1925 7.157897 
 1926 7.271534 
 1927 7.591002 
 1928 7.366596 
 1929 7.117057 
 1930 7.682063 
 1931  7.90196 
 1932 7.858227 
 1933 7.213219 
 1934 7.304888 
 1935 7.517353 
 1936  7.94353 
 1937 7.642859 
 1938 7.827515 
 1939 7.488829 
 1940 7.523391 
 1941 7.833905 
 1942  7.42495 
 1943 7.658608 
 1944 7.162314 
 1945  7.74095 
 1946 7.245343 
 1947 7.099082 
 1948 7.669515 
 1949 6.908273 
 1950 7.017939 
 1951 7.237393 
 1952 7.306597 
 1953  7.99831 
 1954 8.055817 
 1955 7.346125 
 1956 7.293233 
 1957 7.812182 
 1958 7.308995 
 1959 7.442505 
 1960 7.873838 
 1961 7.515775 
 1962 7.410015 
 1963 7.110183 
 1964 7.281657 
 1965 6.867822 
 1966 7.449696 
 1967 7.396806 
 1968 6.898838 
 1969 7.260013 
 1970 7.099854 
 1971  7.61077 
 1972 6.989958 
 1973 7.890626 
 1974 7.580353 
 1975 7.568386 
 1976 7.104658 
 1977 7.630601 
 1978 6.479593 
 1979 7.030251 
 1980 7.439127 
 1981 7.818851 
 1982 7.157169 
 1983 7.468653 
 1984   7.7389 
 1985 7.420447 
 1986  7.01284 
 1987 7.121982 
 1988 8.048256 
 1989 7.865315 
 1990 8.161283 
 1991 8.260771 
 1992 6.829034 
 1993 7.785946 
 1994 7.955132 
 1995 8.019575 
 1996 7.447217 
 1997 7.766425 
 1998 8.565945 
 1999 7.625525 
 2000 8.027372 
 2001 8.394425 
 2002 8.279985 
 2003 8.484739 
 2004 7.834902 
 2005 8.334642 
 2006 8.348649 
 2007 8.502182 
 2008 8.151538 
 2009 8.061087 
 2010 8.393384 
 2011 8.511142 
 2012  8.61093 
 2013 8.638767 
 2014 8.425087 
 2015 8.151815 
 2016 9.069977 
 2017 8.341328 
 2018 8.216637