# year value 1901 18.96031 1902 18.30377 1903 18.19029 1904 18.34224 1905 18.58145 1906 18.80807 1907 18.20458 1908 18.4976 1909 18.61023 1910 18.60235 1911 18.67217 1912 18.12535 1913 18.31994 1914 18.62003 1915 18.44037 1916 18.57706 1917 18.63559 1918 18.47457 1919 18.63799 1920 18.67524 1921 19.01378 1922 18.83251 1923 18.69679 1924 18.74589 1925 18.76797 1926 18.51993 1927 18.81844 1928 18.55287 1929 18.71737 1930 19.06105 1931 19.18911 1932 18.89457 1933 18.95189 1934 18.78517 1935 18.97299 1936 19.06984 1937 19.1566 1938 19.12563 1939 19.21615 1940 18.87198 1941 18.90642 1942 18.84383 1943 18.95223 1944 18.99314 1945 18.7938 1946 18.95229 1947 18.82573 1948 19.08505 1949 18.89853 1950 18.65143 1951 18.81858 1952 19.194 1953 19.16288 1954 18.90785 1955 19.21813 1956 18.55855 1957 18.63762 1958 18.63459 1959 18.97525 1960 18.82097 1961 19.03479 1962 18.70255 1963 18.8772 1964 18.64577 1965 18.48435 1966 18.94397 1967 18.74437 1968 18.44146 1969 18.52843 1970 18.94434 1971 18.71623 1972 18.51734 1973 18.95723 1974 18.71783 1975 19.0757 1976 18.45813 1977 18.84037 1978 18.57383 1979 18.74776 1980 18.8807 1981 19.17745 1982 18.65746 1983 19.01533 1984 18.90225 1985 18.71048 1986 18.86398 1987 18.84006 1988 19.46128 1989 19.13373 1990 19.17413 1991 19.35259 1992 18.34811 1993 18.54895 1994 19.42432 1995 19.43964 1996 19.13032 1997 19.19966 1998 19.81187 1999 19.56771 2000 19.53619 2001 19.64945 2002 19.61528 2003 19.52119 2004 19.11776 2005 19.69601 2006 19.89723 2007 19.84036 2008 19.74752 2009 19.23943 2010 20.16747 2011 19.88844 2012 20.22903 2013 19.74444 2014 19.626 2015 19.92268 2016 20.14721 2017 19.95103 2018 20.15748