# year    value 
 1901 20.59419 
 1902 20.28691 
 1903 20.17633 
 1904 20.11108 
 1905 20.18928 
 1906 20.35158 
 1907 20.01334 
 1908 19.93391 
 1909       20 
 1910 20.14478 
 1911 19.96239 
 1912 20.32373 
 1913 20.52222 
 1914 19.84643 
 1915 20.21711 
 1916 20.38769 
 1917 19.89267 
 1918 20.19453 
 1919 20.22198 
 1920  20.4108 
 1921 19.89797 
 1922 20.04079 
 1923  20.2012 
 1924 19.92646 
 1925 20.45409 
 1926  20.7355 
 1927 20.46908 
 1928 20.28902 
 1929 20.46036 
 1930  20.1142 
 1931 20.09568 
 1932 20.55969 
 1933 20.32692 
 1934 19.80203 
 1935 20.01817 
 1936 20.43169 
 1937 20.29667 
 1938 20.49629 
 1939 20.24885 
 1940 20.25421 
 1941 20.56333 
 1942 20.71822 
 1943 20.66083 
 1944 21.15344 
 1945 21.03095 
 1946 20.58712 
 1947 20.22045 
 1948 20.41915 
 1949 20.00541 
 1950  19.7584 
 1951 20.47045 
 1952 19.89163 
 1953 20.58372 
 1954 20.06956 
 1955   20.108 
 1956 19.72264 
 1957 20.11399 
 1958 20.34007 
 1959 20.18222 
 1960  20.1074 
 1961 20.75292 
 1962 20.96527 
 1963 20.32431 
 1964 20.08984 
 1965 20.69184 
 1966 20.45661 
 1967 20.51974 
 1968 20.62904 
 1969 21.02724 
 1970 20.73176 
 1971 20.53853 
 1972  20.4831 
 1973 19.92602 
 1974 19.80993 
 1975 19.92295 
 1976 20.13534 
 1977 20.46861 
 1978 20.46952 
 1979  20.0391 
 1980 20.01321 
 1981 19.92276 
 1982 20.48256 
 1983 20.42797 
 1984 20.45496 
 1985 20.35113 
 1986 20.30183 
 1987 20.89032 
 1988 20.66892 
 1989 20.28957 
 1990 20.57537 
 1991 20.31291 
 1992 20.09378 
 1993 20.26275 
 1994 20.89079 
 1995 20.78737 
 1996 20.32401 
 1997  20.9232 
 1998 20.35717 
 1999  20.3517 
 2000 20.31213 
 2001 20.37912 
 2002 20.80371 
 2003 20.78954 
 2004 20.87463 
 2005 20.12436 
 2006 20.61043 
 2007 20.48034 
 2008 20.71685 
 2009 20.85237 
 2010 20.45585 
 2011 20.82566 
 2012 21.12786 
 2013 20.47965 
 2014 21.29338 
 2015 20.89264 
 2016 20.65564 
 2017 20.68529 
 2018 20.87729