# year    value 
 1901 21.01254 
 1902 20.50418 
 1903 20.54111 
 1904 20.79942 
 1905 20.70467 
 1906  20.3155 
 1907 20.23476 
 1908  20.6302 
 1909  20.4947 
 1910 20.65853 
 1911 20.24025 
 1912  20.2407 
 1913 20.87944 
 1914 20.99158 
 1915 20.48755 
 1916 19.69991 
 1917 20.19351 
 1918 20.64383 
 1919 20.46212 
 1920  20.0176 
 1921 19.66372 
 1922  20.2322 
 1923 20.18131 
 1924 20.15177 
 1925 20.38142 
 1926 20.71576 
 1927 20.23686 
 1928 20.15273 
 1929 20.31276 
 1930 20.55004 
 1931 20.09614 
 1932 20.68029 
 1933  20.1682 
 1934 20.65932 
 1935 20.62027 
 1936 20.47734 
 1937 20.96858 
 1938  20.3731 
 1939 21.17489 
 1940 21.02908 
 1941 21.00058 
 1942 20.05682 
 1943 20.51762 
 1944 21.27841 
 1945 21.09022 
 1946  20.3382 
 1947 20.38424 
 1948 20.56938 
 1949 20.20224 
 1950 20.54263 
 1951 20.95633 
 1952  19.8857 
 1953 20.45136 
 1954 20.20789 
 1955 19.94196 
 1956 19.80185 
 1957 19.99274 
 1958  20.9728 
 1959 20.55801 
 1960  20.3577 
 1961 21.12008 
 1962 20.16939 
 1963 21.40286 
 1964 20.36644 
 1965 21.23724 
 1966 20.73938 
 1967 20.31081 
 1968  21.2059 
 1969 20.91205 
 1970 20.69484 
 1971 20.43784 
 1972 20.91384 
 1973 20.09205 
 1974 20.26495 
 1975 20.23463 
 1976 20.21772 
 1977 20.68726 
 1978 20.32016 
 1979  20.8276 
 1980 20.33823 
 1981 20.21846 
 1982  20.7758 
 1983 19.97333 
 1984 19.83879 
 1985 20.47581 
 1986 20.84304 
 1987 21.08764 
 1988 20.22563 
 1989 20.57723 
 1990 20.37768 
 1991 20.58336 
 1992 20.52239 
 1993 20.61019 
 1994 21.01381 
 1995 20.83509 
 1996 20.45319 
 1997 21.39925 
 1998 21.21296 
 1999 20.23647 
 2000 20.29865 
 2001   20.821 
 2002  20.5825 
 2003 20.57478 
 2004 20.77176 
 2005 21.01449 
 2006 21.25265 
 2007 19.87108 
 2008 20.90453 
 2009 20.85741 
 2010 20.53267 
 2011 20.88097 
 2012 20.87533 
 2013 20.66807 
 2014  21.3808 
 2015 21.84227 
 2016 20.93907 
 2017 21.42461 
 2018 20.17289