# year    value 
 1901 16.92426 
 1902 16.88978 
 1903 16.83926 
 1904 16.88384 
 1905 16.99886 
 1906 17.00504 
 1907 16.67599 
 1908  17.0144 
 1909 16.80613 
 1910 16.83194 
 1911 16.88937 
 1912 16.92142 
 1913 16.76125 
 1914 17.15675 
 1915  17.0448 
 1916 16.99367 
 1917 16.95502 
 1918 16.90117 
 1919 17.21133 
 1920 16.89583 
 1921 16.93373 
 1922  17.1101 
 1923 17.16784 
 1924 17.24048 
 1925 17.11165 
 1926 17.24305 
 1927 17.41506 
 1928  17.1121 
 1929 17.14117 
 1930  17.2591 
 1931 17.19313 
 1932 17.32421 
 1933 17.38073 
 1934   17.106 
 1935 17.24714 
 1936 17.00617 
 1937 17.21716 
 1938 17.04218 
 1939 17.40187 
 1940 17.23293 
 1941 17.65766 
 1942 17.39464 
 1943 17.27352 
 1944  17.5378 
 1945 17.63961 
 1946 17.45324 
 1947  17.3955 
 1948 17.36698 
 1949 16.97281 
 1950 17.07732 
 1951 17.04758 
 1952 17.10678 
 1953 17.48792 
 1954 16.85769 
 1955 17.07932 
 1956 16.78378 
 1957 16.85662 
 1958 17.12143 
 1959 17.20826 
 1960 17.22415 
 1961   17.439 
 1962  17.1016 
 1963 17.12874 
 1964 16.86965 
 1965 16.72733 
 1966 17.17046 
 1967 17.02662 
 1968 16.76257 
 1969 16.83658 
 1970   16.988 
 1971 16.95496 
 1972 17.10909 
 1973 17.08751 
 1974 16.81745 
 1975 17.15336 
 1976 16.42233 
 1977  17.0902 
 1978 17.22589 
 1979 17.09012 
 1980 17.04706 
 1981 17.39232 
 1982  16.9329 
 1983 17.08858 
 1984 17.10122 
 1985 17.06519 
 1986 17.04738 
 1987 17.06875 
 1988 17.36628 
 1989 16.90767 
 1990 17.27029 
 1991 17.28845 
 1992 16.68988 
 1993 16.73936 
 1994 17.40812 
 1995 17.33418 
 1996 16.94864 
 1997 17.26647 
 1998 17.56842 
 1999 17.41011 
 2000 17.42686 
 2001 17.52915 
 2002 17.66123 
 2003   17.311 
 2004 17.18298 
 2005 17.71116 
 2006 17.87404 
 2007 17.64308 
 2008 17.44134 
 2009 17.67883 
 2010 17.82149 
 2011  17.7014 
 2012 17.74357 
 2013 17.93483 
 2014  17.5676 
 2015 17.47964 
 2016 18.02393 
 2017 17.82614 
 2018 18.06428