# year    value 
 1901 21.02737 
 1902 20.04054 
 1903 19.91244 
 1904 19.90477 
 1905 20.27085 
 1906 19.99262 
 1907 19.58739 
 1908 19.84311 
 1909 20.73545 
 1910 20.81351 
 1911 20.40617 
 1912 19.74795 
 1913 20.62578 
 1914  20.6567 
 1915 19.45816 
 1916 20.18921 
 1917 20.61592 
 1918 21.18717 
 1919 21.06671 
 1920 20.22374 
 1921 20.95788 
 1922 21.28934 
 1923 20.43884 
 1924 20.46668 
 1925 20.77849 
 1926  20.8894 
 1927 20.10834 
 1928 20.06379 
 1929  21.0377 
 1930 21.19299 
 1931 21.72487 
 1932 20.93109 
 1933 21.76195 
 1934 21.82771 
 1935  21.0976 
 1936 22.36045 
 1937 21.51832 
 1938 21.16844 
 1939 21.13715 
 1940 21.44179 
 1941 20.52348 
 1942 20.61666 
 1943 20.81495 
 1944 20.14083 
 1945 20.21154 
 1946 20.99197 
 1947 20.61885 
 1948 20.69557 
 1949 20.80101 
 1950 19.97735 
 1951 20.24593 
 1952 21.09597 
 1953 20.97502 
 1954 20.86918 
 1955 20.71138 
 1956 21.09924 
 1957 20.97326 
 1958 21.16223 
 1959 21.40918 
 1960 21.31322 
 1961 21.71875 
 1962  20.3937 
 1963 20.97897 
 1964 20.70393 
 1965  20.2165 
 1966 20.87764 
 1967  20.7149 
 1968 20.26776 
 1969 20.90964 
 1970 21.54142 
 1971  21.0316 
 1972 20.81778 
 1973 20.92512 
 1974 20.98544 
 1975 20.41388 
 1976 20.47642 
 1977 21.48011 
 1978  21.0005 
 1979 20.70239 
 1980 21.27007 
 1981 21.27673 
 1982 20.50088 
 1983 21.03269 
 1984 21.11544 
 1985 20.98385 
 1986 21.23178 
 1987 20.74641 
 1988 21.83758 
 1989 20.87601 
 1990 21.27626 
 1991 21.02845 
 1992 20.09137 
 1993 19.83705 
 1994 21.84364 
 1995 20.91911 
 1996 21.62908 
 1997 20.71242 
 1998 21.17659 
 1999 20.60305 
 2000 21.37189 
 2001 21.71062 
 2002 21.65753 
 2003 22.00105 
 2004 20.53532 
 2005 21.29554 
 2006 22.29796 
 2007 21.95016 
 2008 21.30714 
 2009 20.87085 
 2010 21.35164 
 2011 21.87736 
 2012 22.34057 
 2013 21.96518 
 2014 21.43345 
 2015 22.07005 
 2016 22.03966 
 2017 21.69284 
 2018 22.20829