# year value 1901 14.68821 1902 14.31576 1903 14.33945 1904 14.2384 1905 14.29269 1906 14.71345 1907 14.06482 1908 14.18681 1909 14.08133 1910 14.41809 1911 14.16118 1912 14.34465 1913 14.23496 1914 14.4647 1915 14.54346 1916 14.24823 1917 13.9235 1918 14.06938 1919 14.3191 1920 14.58615 1921 14.69152 1922 14.59408 1923 14.16884 1924 14.44818 1925 14.53836 1926 14.47539 1927 14.37565 1928 14.37247 1929 14.25704 1930 14.58149 1931 14.42982 1932 14.55197 1933 14.37181 1934 14.5255 1935 14.42591 1936 14.5549 1937 14.50913 1938 15.03748 1939 14.5717 1940 14.64956 1941 14.59493 1942 14.60913 1943 14.68539 1944 14.70881 1945 14.54351 1946 14.71238 1947 14.71323 1948 14.55865 1949 14.53942 1950 14.48255 1951 14.58203 1952 14.51352 1953 14.94518 1954 14.35191 1955 14.25066 1956 14.04274 1957 14.36836 1958 14.70881 1959 14.75162 1960 14.06534 1961 14.77936 1962 14.74665 1963 14.38873 1964 14.25585 1965 14.37633 1966 14.46894 1967 14.70062 1968 14.68721 1969 14.47979 1970 14.59019 1971 14.31384 1972 14.40662 1973 15.05225 1974 14.48837 1975 14.81888 1976 14.2951 1977 15.07144 1978 14.66486 1979 14.54902 1980 14.88361 1981 15.09577 1982 14.59838 1983 14.84584 1984 14.77148 1985 14.75946 1986 15.02269 1987 14.76031 1988 15.19243 1989 14.91615 1990 15.52095 1991 15.15688 1992 14.87522 1993 15.03485 1994 15.13605 1995 15.20686 1996 14.78522 1997 15.03827 1998 15.561 1999 15.11194 2000 15.47516 2001 15.48335 2002 15.43715 2003 15.343 2004 15.32082 2005 15.69366 2006 15.4045 2007 15.71187 2008 15.47834 2009 15.32508 2010 15.95116 2011 15.44108 2012 15.69611 2013 15.36968 2014 15.65182 2015 15.684 2016 16.23975 2017 15.81345 2018 15.56737