# year    value 
 1901 21.39582 
 1902  21.2586 
 1903 21.08515 
 1904 21.11666 
 1905 20.78657 
 1906 21.15469 
 1907 21.26087 
 1908 21.05873 
 1909 21.25756 
 1910 21.10008 
 1911 20.99574 
 1912 21.24758 
 1913 21.10125 
 1914 21.27799 
 1915 21.30882 
 1916 21.10527 
 1917  21.1882 
 1918 20.83243 
 1919 21.58528 
 1920 21.07622 
 1921 21.14412 
 1922  21.4908 
 1923 21.20178 
 1924 21.60725 
 1925 20.64643 
 1926 21.47779 
 1927 21.24849 
 1928 21.50273 
 1929 21.25173 
 1930 20.99738 
 1931 21.73055 
 1932 21.47081 
 1933 21.50011 
 1934 21.13833 
 1935 21.48975 
 1936 21.63202 
 1937 21.51157 
 1938 21.80595 
 1939 21.65429 
 1940 21.63544 
 1941 21.90354 
 1942 21.62992 
 1943 21.24345 
 1944 21.43534 
 1945   21.186 
 1946 21.42144 
 1947 21.63424 
 1948 21.51047 
 1949 21.03345 
 1950 21.05763 
 1951 21.17469 
 1952 21.31652 
 1953 21.67464 
 1954 21.34159 
 1955 21.49507 
 1956 21.27135 
 1957 20.87552 
 1958 21.66295 
 1959 21.45853 
 1960 21.53474 
 1961 21.31359 
 1962 21.00123 
 1963 21.75912 
 1964 21.17242 
 1965 21.27753 
 1966 21.52327 
 1967 21.08644 
 1968 20.86949 
 1969 21.77221 
 1970 21.80429 
 1971 21.12408 
 1972 21.00515 
 1973 21.63565 
 1974 20.92477 
 1975 20.91416 
 1976 20.98701 
 1977 21.45338 
 1978 21.53195 
 1979 21.89787 
 1980 21.59791 
 1981  21.1651 
 1982 21.48478 
 1983 21.00397 
 1984  21.1622 
 1985 21.51276 
 1986 21.43859 
 1987  21.5543 
 1988 21.84449 
 1989  20.7432 
 1990 21.56274 
 1991 21.99513 
 1992 20.93365 
 1993 21.28069 
 1994 21.62484 
 1995  21.2766 
 1996 21.97594 
 1997 21.63544 
 1998 22.13216 
 1999 22.08946 
 2000 21.59842 
 2001 21.71099 
 2002 21.98539 
 2003 22.22552 
 2004 22.06628 
 2005 22.11064 
 2006 22.45011 
 2007 21.80279 
 2008 21.52016 
 2009 22.34193 
 2010 22.78813 
 2011 22.08174 
 2012 21.66071 
 2013 22.20871 
 2014 21.80948 
 2015 21.87885 
 2016 22.06078 
 2017 22.25015 
 2018 22.11975