# year value 1901 18.20763 1902 18.28236 1903 17.12496 1904 17.72092 1905 16.69833 1906 17.82642 1907 17.32143 1908 17.5508 1909 17.80419 1910 17.57785 1911 17.57205 1912 17.89854 1913 17.34606 1914 17.99857 1915 18.1519 1916 18.09409 1917 17.56559 1918 17.52311 1919 17.95794 1920 17.39568 1921 18.17378 1922 18.39863 1923 18.12915 1924 17.9825 1925 18.11118 1926 17.78932 1927 17.51442 1928 18.03764 1929 18.68844 1930 18.08213 1931 17.97442 1932 18.31725 1933 17.62597 1934 17.33689 1935 17.98034 1936 17.2051 1937 17.24596 1938 18.89355 1939 17.60055 1940 18.02994 1941 19.0215 1942 18.60305 1943 17.96814 1944 18.48916 1945 17.95404 1946 18.31178 1947 18.733 1948 18.44675 1949 17.97473 1950 18.00855 1951 17.41373 1952 18.08544 1953 18.49836 1954 17.5546 1955 17.61796 1956 18.08266 1957 17.26706 1958 18.51847 1959 18.3808 1960 17.25663 1961 18.72892 1962 18.14574 1963 18.28243 1964 18.41589 1965 17.86641 1966 18.06638 1967 18.09158 1968 18.05265 1969 18.16194 1970 17.75636 1971 17.95975 1972 18.10828 1973 18.5756 1974 18.41506 1975 18.26465 1976 17.43121 1977 18.69931 1978 18.36362 1979 17.67189 1980 18.2149 1981 18.79184 1982 18.08398 1983 17.96268 1984 18.31039 1985 18.08704 1986 18.12497 1987 18.09744 1988 18.10376 1989 18.17758 1990 18.3242 1991 18.03176 1992 18.05262 1993 18.06049 1994 18.63675 1995 18.23674 1996 17.9862 1997 18.91321 1998 18.87912 1999 18.72822 2000 19.33619 2001 19.35446 2002 19.38707 2003 18.33724 2004 19.65654 2005 19.02279 2006 19.25207 2007 19.3901 2008 20.02915 2009 19.44494 2010 19.03039 2011 18.84861 2012 19.1756 2013 19.43564 2014 18.83627 2015 19.14365 2016 19.86441 2017 19.49349 2018 20.09915