# year value 1901 19.12558 1902 19.21942 1903 18.96993 1904 18.90712 1905 19.19351 1906 18.96367 1907 18.94781 1908 18.96004 1909 18.87006 1910 18.60715 1911 18.89881 1912 19.02694 1913 19.20585 1914 19.08219 1915 19.06525 1916 19.01313 1917 18.60169 1918 19.05241 1919 19.00014 1920 19.29857 1921 18.83748 1922 19.04629 1923 19.24035 1924 18.89315 1925 19.13651 1926 19.60462 1927 19.41724 1928 19.39008 1929 19.30025 1930 19.02537 1931 19.04566 1932 19.42241 1933 19.06529 1934 18.89783 1935 19.112 1936 19.18526 1937 19.15109 1938 19.19575 1939 19.10188 1940 19.16445 1941 19.41617 1942 19.19674 1943 19.27262 1944 19.65607 1945 19.53478 1946 19.41461 1947 19.05272 1948 19.14618 1949 19.05999 1950 18.78172 1951 19.30763 1952 19.02252 1953 19.35662 1954 19.09563 1955 18.85178 1956 18.74104 1957 19.38752 1958 19.33031 1959 19.20516 1960 19.19512 1961 19.42249 1962 19.31332 1963 19.37106 1964 18.84404 1965 19.21959 1966 19.26549 1967 19.21701 1968 19.17378 1969 19.61244 1970 19.3073 1971 18.94087 1972 19.48912 1973 19.15864 1974 18.91368 1975 18.87202 1976 19.04074 1977 19.4611 1978 19.3241 1979 19.27258 1980 19.18713 1981 19.04878 1982 19.33373 1983 19.41988 1984 19.3723 1985 19.33419 1986 19.30981 1987 19.72509 1988 19.38879 1989 19.21505 1990 19.51187 1991 19.35213 1992 19.27769 1993 19.4689 1994 19.73159 1995 19.7373 1996 19.38328 1997 19.94758 1998 19.60759 1999 19.21726 2000 19.41692 2001 19.47873 2002 19.82603 2003 19.83898 2004 19.83129 2005 19.59469 2006 19.58637 2007 19.67941 2008 19.7345 2009 19.89909 2010 19.69486 2011 19.51518 2012 19.91715 2013 19.67168 2014 19.96408 2015 20.12084 2016 19.8138 2017 19.83027 2018 19.86786