# year    value 
 1901 20.48013 
 1902 20.57357 
 1903 20.29709 
 1904 20.30927 
 1905 19.96485 
 1906 20.33113 
 1907  20.1422 
 1908 20.65278 
 1909 20.36189 
 1910 20.22735 
 1911 20.19897 
 1912 20.25866 
 1913 19.96763 
 1914 20.21382 
 1915 20.24862 
 1916 20.59967 
 1917 19.94846 
 1918 20.08696 
 1919 20.44665 
 1920 20.10159 
 1921 20.53544 
 1922  20.6023 
 1923 20.48557 
 1924 20.61333 
 1925  20.4024 
 1926 20.28744 
 1927 20.27334 
 1928 20.81543 
 1929 20.56523 
 1930 20.38078 
 1931 20.57556 
 1932 20.47177 
 1933 20.55465 
 1934 20.42045 
 1935  20.6925 
 1936 20.52138 
 1937 20.52843 
 1938 20.63478 
 1939 20.42326 
 1940  20.3858 
 1941 20.89154 
 1942 20.81447 
 1943 20.39281 
 1944 20.52181 
 1945 20.45622 
 1946 20.75869 
 1947 20.67532 
 1948 20.53115 
 1949 20.44725 
 1950 20.40774 
 1951 20.27077 
 1952 20.67522 
 1953  20.7777 
 1954 20.69772 
 1955 20.62861 
 1956 20.56557 
 1957 20.16151 
 1958 21.18821 
 1959 20.61286 
 1960 20.51957 
 1961 20.62029 
 1962 20.38229 
 1963  20.5219 
 1964 20.76351 
 1965 20.18341 
 1966 20.63532 
 1967 20.27356 
 1968 19.91044 
 1969 20.95987 
 1970 20.41626 
 1971 20.31291 
 1972 20.46903 
 1973 21.02832 
 1974 20.45756 
 1975 20.63647 
 1976  20.1626 
 1977 20.58409 
 1978 20.70604 
 1979 20.70139 
 1980 21.01892 
 1981 20.55399 
 1982 20.47766 
 1983 20.56359 
 1984 20.86764 
 1985 20.58414 
 1986 20.58853 
 1987 20.61814 
 1988 20.86385 
 1989 20.46563 
 1990 20.54869 
 1991 21.00468 
 1992 20.62893 
 1993 20.51988 
 1994 20.90075 
 1995 20.92488 
 1996 20.68562 
 1997 20.52992 
 1998 21.45046 
 1999  21.0856 
 2000 20.96639 
 2001 21.08389 
 2002  21.3931 
 2003 21.31938 
 2004 21.30971 
 2005  21.1986 
 2006 21.15876 
 2007 21.20738 
 2008 21.13727 
 2009 21.27728 
 2010 21.66882 
 2011  21.0666 
 2012 21.27049 
 2013 21.37852 
 2014 21.23746 
 2015 21.33693 
 2016 21.73092 
 2017 21.24135 
 2018  21.4132