# year    value 
 1901 20.87517 
 1902 21.05162 
 1903 20.96794 
 1904  20.8229 
 1905 20.92245 
 1906  21.0515 
 1907  20.7223 
 1908 20.71295 
 1909 20.94487 
 1910 21.05306 
 1911 20.55842 
 1912 20.72572 
 1913 20.72891 
 1914 20.96873 
 1915 21.11437 
 1916 20.78844 
 1917 20.57059 
 1918 20.63324 
 1919 20.84354 
 1920 20.66815 
 1921 20.66989 
 1922 20.63864 
 1923 20.56175 
 1924 20.74292 
 1925 20.58453 
 1926  20.9346 
 1927 20.66884 
 1928  20.7643 
 1929 20.54443 
 1930 20.90204 
 1931 20.90594 
 1932 20.72737 
 1933 20.67571 
 1934 20.98667 
 1935 20.68561 
 1936 20.96202 
 1937 20.83819 
 1938 20.90667 
 1939 20.79614 
 1940 21.12255 
 1941 21.01211 
 1942 21.08377 
 1943 20.60538 
 1944 21.19457 
 1945 21.09898 
 1946 20.70251 
 1947 20.97774 
 1948 20.99827 
 1949 20.80756 
 1950   20.779 
 1951  20.6866 
 1952 20.78493 
 1953 20.65387 
 1954 20.71321 
 1955 20.80476 
 1956 20.40061 
 1957 20.90281 
 1958 21.14742 
 1959 21.16606 
 1960  20.7745 
 1961 20.94445 
 1962 20.91211 
 1963 20.94711 
 1964 20.78138 
 1965 21.00583 
 1966 21.04311 
 1967 20.70292 
 1968 20.79468 
 1969 21.25022 
 1970 21.13194 
 1971 20.66548 
 1972 21.11448 
 1973 21.19383 
 1974 20.67686 
 1975 20.82526 
 1976 20.53849 
 1977 20.99268 
 1978 20.79729 
 1979 21.03681 
 1980 21.04315 
 1981 20.84852 
 1982 21.03777 
 1983 21.08147 
 1984 20.90046 
 1985 20.91055 
 1986 21.09069 
 1987 21.40251 
 1988 21.26218 
 1989  20.8284 
 1990 21.05001 
 1991 21.31888 
 1992 21.18137 
 1993 21.29476 
 1994  21.0947 
 1995 21.57866 
 1996 21.36279 
 1997 21.41629 
 1998 21.85976 
 1999 21.35156 
 2000 21.14712 
 2001 21.41688 
 2002 21.51436 
 2003 21.51679 
 2004 21.31856 
 2005 21.79444 
 2006 21.43286 
 2007   21.133 
 2008 21.37272 
 2009 21.61915 
 2010 21.48197 
 2011 21.37116 
 2012 21.31626 
 2013 21.61802 
 2014 21.56061 
 2015 21.89808 
 2016 21.75206 
 2017  21.9252 
 2018 21.40613