# year value 1901 15.2606 1902 14.48062 1903 14.29169 1904 16.20661 1905 15.99389 1906 14.94703 1907 14.67498 1908 14.9993 1909 13.97629 1910 14.26465 1911 16.13945 1912 14.09918 1913 13.47657 1914 14.12164 1915 14.77696 1916 14.07933 1917 15.70433 1918 13.94124 1919 14.42124 1920 14.37125 1921 15.6918 1922 15.03943 1923 14.81802 1924 14.13235 1925 14.78297 1926 13.96239 1927 15.47945 1928 16.61215 1929 15.73747 1930 15.47537 1931 15.48237 1932 15.40802 1933 14.6945 1934 15.16621 1935 15.89861 1936 14.77209 1937 15.53549 1938 15.51508 1939 15.16001 1940 14.00014 1941 14.98586 1942 15.33309 1943 15.69277 1944 15.6869 1945 16.35956 1946 15.82974 1947 17.02898 1948 14.25178 1949 15.26842 1950 17.22071 1951 15.43306 1952 16.99145 1953 15.10429 1954 14.53055 1955 14.757 1956 14.47946 1957 15.28935 1958 15.41223 1959 15.57551 1960 14.72074 1961 15.09674 1962 15.07761 1963 15.40914 1964 15.85047 1965 14.50181 1966 14.61158 1967 15.47626 1968 14.41294 1969 14.48765 1970 15.40625 1971 15.69763 1972 14.71897 1973 15.71988 1974 15.05626 1975 15.0215 1976 15.28192 1977 14.56845 1978 14.10968 1979 15.00396 1980 14.49337 1981 15.15228 1982 16.1706 1983 16.6863 1984 14.75751 1985 15.41303 1986 15.44514 1987 15.20809 1988 15.85138 1989 15.46126 1990 15.85057 1991 16.23966 1992 16.673 1993 15.86575 1994 17.42341 1995 15.98267 1996 15.53719 1997 15.75978 1998 16.68629 1999 16.16698 2000 16.41751 2001 16.32813 2002 16.68825 2003 18.4436 2004 16.11892 2005 16.10372 2006 16.62099 2007 16.48314 2008 16.57529 2009 16.8849 2010 16.76393 2011 16.35146 2012 17.51866 2013 17.10641 2014 15.98499 2015 18.3694 2016 16.90186 2017 18.10383 2018 18.07128