# year    value 
 1901 17.37567 
 1902 16.52839 
 1903 16.36192 
 1904 18.25185 
 1905 18.07464 
 1906 16.98089 
 1907 16.72335 
 1908 17.08531 
 1909 16.07902 
 1910 16.36521 
 1911  18.2158 
 1912   16.217 
 1913 15.54246 
 1914 16.23147 
 1915 16.84792 
 1916 16.16826 
 1917 17.84499 
 1918 16.02961 
 1919 16.46035 
 1920 16.45271 
 1921 17.77354 
 1922 17.12266 
 1923  16.8618 
 1924 16.22788 
 1925 16.85329 
 1926 16.01436 
 1927 17.57207 
 1928 18.62379 
 1929 17.78984 
 1930 17.56471 
 1931 17.57577 
 1932 17.54069 
 1933 16.78799 
 1934 17.27359 
 1935 17.98033 
 1936 16.87446 
 1937 17.60037 
 1938 17.61363 
 1939 17.27566 
 1940 16.07625 
 1941 17.04102 
 1942 17.38302 
 1943 17.73431 
 1944 17.73973 
 1945 18.41139 
 1946 17.96324 
 1947 19.10697 
 1948 16.36167 
 1949  17.3013 
 1950 19.27756 
 1951 17.49052 
 1952 19.04526 
 1953  17.1703 
 1954 16.61659 
 1955 16.84234 
 1956 16.54373 
 1957 17.42066 
 1958 17.46812 
 1959 17.68485 
 1960 16.79786 
 1961 17.12835 
 1962  17.0848 
 1963 17.50625 
 1964 17.92763 
 1965 16.49455 
 1966 16.66784 
 1967 17.55471 
 1968 16.53141 
 1969 16.58272 
 1970 17.46401 
 1971 17.72465 
 1972 16.79946 
 1973 17.77344 
 1974 17.07511 
 1975 17.12182 
 1976  17.4055 
 1977 16.65569 
 1978 16.12774 
 1979 17.07804 
 1980  16.5134 
 1981 17.19721 
 1982  18.2081 
 1983 18.75235 
 1984  16.7488 
 1985 17.39682 
 1986 17.49543 
 1987 17.24708 
 1988 17.88505 
 1989 17.52635 
 1990 17.90496 
 1991 18.26812 
 1992 18.77695 
 1993 17.91642 
 1994  19.4662 
 1995  18.0918 
 1996 17.57867 
 1997 17.85102 
 1998 18.70535 
 1999 18.22973 
 2000 18.48262 
 2001 18.38709 
 2002 18.75902 
 2003 20.44497 
 2004 18.15999 
 2005 18.13783 
 2006 18.67956 
 2007 18.60373 
 2008  18.6492 
 2009 18.92568 
 2010  18.8348 
 2011 18.43309 
 2012  19.5793 
 2013 19.17872 
 2014 18.08055 
 2015 20.38657 
 2016 18.94755 
 2017 20.13903 
 2018 20.13272