# year value 1901 18.58174 1902 18.7454 1903 18.54353 1904 18.33264 1905 18.52387 1906 18.47291 1907 18.26952 1908 18.4651 1909 18.32928 1910 18.19656 1911 18.25393 1912 18.56226 1913 18.54077 1914 18.63049 1915 18.68425 1916 18.3104 1917 18.08203 1918 18.2524 1919 18.53947 1920 18.4557 1921 18.34189 1922 18.55514 1923 18.59073 1924 18.52132 1925 18.5783 1926 18.89743 1927 18.65987 1928 18.74571 1929 18.54773 1930 18.553 1931 18.56001 1932 18.6546 1933 18.50743 1934 18.51426 1935 18.52707 1936 18.58882 1937 18.61668 1938 18.60519 1939 18.62628 1940 18.76094 1941 18.91528 1942 18.69408 1943 18.66015 1944 18.898 1945 18.81955 1946 18.72054 1947 18.60771 1948 18.64194 1949 18.56828 1950 18.37445 1951 18.44891 1952 18.59406 1953 18.65994 1954 18.46324 1955 18.30739 1956 18.15317 1957 18.65791 1958 18.82774 1959 18.69341 1960 18.52245 1961 18.81905 1962 18.50208 1963 18.65265 1964 18.41141 1965 18.61765 1966 18.72861 1967 18.63794 1968 18.40753 1969 19.04276 1970 18.80724 1971 18.40161 1972 18.82314 1973 18.82567 1974 18.32224 1975 18.38301 1976 18.38928 1977 18.78381 1978 18.61353 1979 18.62994 1980 18.80302 1981 18.59526 1982 18.76292 1983 19.17493 1984 18.71808 1985 18.70278 1986 18.8342 1987 19.21159 1988 18.90263 1989 18.68931 1990 18.88115 1991 18.91121 1992 18.96535 1993 18.98469 1994 19.07395 1995 19.1259 1996 18.77116 1997 19.17786 1998 19.34813 1999 18.8322 2000 18.82804 2001 19.00172 2002 19.19736 2003 19.20994 2004 19.09384 2005 19.20117 2006 19.13077 2007 18.95991 2008 18.92448 2009 19.20335 2010 19.24901 2011 18.95613 2012 19.17145 2013 19.06132 2014 19.23512 2015 19.47353 2016 19.41251 2017 19.36329 2018 19.16488