# year    value 
 1901 20.42618 
 1902 19.70817 
 1903 19.49087 
 1904  19.6388 
 1905 19.90884 
 1906 20.23243 
 1907  19.5923 
 1908 19.80633 
 1909 20.01602 
 1910 19.96036 
 1911 20.12674 
 1912 19.49217 
 1913 19.75804 
 1914 19.99205 
 1915 19.74447 
 1916 19.96832 
 1917 19.98655 
 1918 19.82068 
 1919 20.00136 
 1920 20.10196 
 1921 20.48609 
 1922 20.17021 
 1923 20.06079 
 1924 20.04616 
 1925 20.17992 
 1926 19.72575 
 1927 20.15548 
 1928 19.80111 
 1929 20.13837 
 1930 20.50878 
 1931 20.66185 
 1932 20.37106 
 1933 20.36287 
 1934 20.20165 
 1935 20.37576 
 1936 20.60367 
 1937 20.66526 
 1938 20.61304 
 1939 20.62817 
 1940  20.2715 
 1941 20.19783 
 1942 20.18189 
 1943 20.39304 
 1944 20.34345 
 1945 20.11393 
 1946 20.25112 
 1947 20.21785 
 1948 20.48375 
 1949 20.33914 
 1950 19.86588 
 1951 20.23717 
 1952  20.6695 
 1953 20.62846 
 1954 20.38877 
 1955 20.73003 
 1956 19.96653 
 1957 20.11055 
 1958 19.90504 
 1959 20.39827 
 1960 20.19052 
 1961 20.39142 
 1962 20.03962 
 1963 20.30861 
 1964 20.04871 
 1965 19.86677 
 1966 20.38124 
 1967  20.1456 
 1968 19.79582 
 1969 19.88229 
 1970 20.30235 
 1971 20.08117 
 1972 19.94659 
 1973 20.41085 
 1974 20.07097 
 1975 20.57928 
 1976 19.99359 
 1977 20.19488 
 1978 19.86343 
 1979 20.11205 
 1980 20.23221 
 1981  20.6798 
 1982 20.01779 
 1983 20.63358 
 1984 20.40684 
 1985 20.03415 
 1986 20.17864 
 1987 20.29715 
 1988 21.01183 
 1989 20.63886 
 1990 20.55292 
 1991 20.82305 
 1992 19.61846 
 1993 19.99482 
 1994 20.72504 
 1995  20.9731 
 1996 20.53698 
 1997 20.61516 
 1998 21.28129 
 1999 21.05505 
 2000   20.859 
 2001 21.00859 
 2002   20.973 
 2003 20.91266 
 2004  20.4002 
 2005 21.12232 
 2006 21.32498 
 2007 21.22102 
 2008 21.14528 
 2009 20.55912 
 2010 21.71577 
 2011  21.3547 
 2012 21.77856 
 2013  21.0886 
 2014 21.00078 
 2015 21.25051 
 2016 21.57931 
 2017 21.24522 
 2018 21.53712