# year    value 
 1901 20.59451 
 1902 20.37707 
 1903 20.27944 
 1904 20.27429 
 1905 20.38038 
 1906 20.55931 
 1907 20.19966 
 1908 20.33094 
 1909 20.41652 
 1910 20.45214 
 1911 20.37357 
 1912 20.20376 
 1913 20.32442 
 1914 20.47065 
 1915 20.51935 
 1916 20.32541 
 1917 20.31878 
 1918 20.24326 
 1919 20.42213 
 1920 20.40104 
 1921 20.55803 
 1922 20.38309 
 1923 20.35908 
 1924 20.42212 
 1925 20.38477 
 1926 20.45547 
 1927 20.46207 
 1928 20.38365 
 1929 20.27124 
 1930 20.60989 
 1931 20.77045 
 1932  20.6116 
 1933 20.46257 
 1934 20.54808 
 1935 20.55425 
 1936  20.6903 
 1937 20.72205 
 1938 20.70607 
 1939 20.69519 
 1940 20.69446 
 1941 20.71902 
 1942 20.56533 
 1943 20.46051 
 1944 20.66146 
 1945 20.63395 
 1946 20.42704 
 1947 20.58622 
 1948 20.65364 
 1949 20.46407 
 1950 20.26738 
 1951 20.52572 
 1952  20.5996 
 1953 20.63995 
 1954 20.52079 
 1955 20.55371 
 1956 20.18762 
 1957 20.54544 
 1958 20.49164 
 1959 20.62934 
 1960  20.5509 
 1961  20.5573 
 1962 20.46784 
 1963 20.57174 
 1964 20.34017 
 1965 20.36502 
 1966 20.66196 
 1967 20.37769 
 1968 20.30176 
 1969 20.55128 
 1970 20.58567 
 1971 20.34873 
 1972 20.49269 
 1973 20.77236 
 1974 20.41838 
 1975 20.57893 
 1976 20.27494 
 1977 20.63352 
 1978 20.24053 
 1979  20.5887 
 1980 20.63585 
 1981 20.78001 
 1982 20.55498 
 1983 20.86461 
 1984 20.65463 
 1985 20.54316 
 1986  20.6047 
 1987 20.90767 
 1988 21.03794 
 1989 20.78092 
 1990 20.90316 
 1991 21.02203 
 1992 20.41106 
 1993 20.76382 
 1994 20.87147 
 1995 21.19237 
 1996  20.8617 
 1997 21.01544 
 1998 21.52471 
 1999 21.03637 
 2000 20.99374 
 2001 21.18307 
 2002 21.26378 
 2003 21.23435 
 2004 20.94126 
 2005 21.37822 
 2006 21.36604 
 2007 21.18444 
 2008 21.20148 
 2009 21.22328 
 2010 21.49984 
 2011 21.38928 
 2012 21.46224 
 2013 21.31663 
 2014 21.34864 
 2015 21.50454 
 2016  21.6367 
 2017 21.49898 
 2018  21.3613