# year value 1901 20.05492 1902 20.224 1903 20.12923 1904 19.99882 1905 20.10169 1906 20.14733 1907 19.83469 1908 19.91519 1909 20.07721 1910 20.16236 1911 19.67634 1912 19.85661 1913 19.9111 1914 20.14977 1915 20.23474 1916 19.85585 1917 19.70166 1918 19.79848 1919 19.97023 1920 19.77782 1921 19.77563 1922 19.82757 1923 19.74715 1924 19.90535 1925 19.7861 1926 20.11985 1927 19.82895 1928 19.92931 1929 19.72939 1930 20.05491 1931 20.04753 1932 19.92042 1933 19.84518 1934 20.15731 1935 19.85803 1936 20.09863 1937 20.02983 1938 20.0551 1939 20.0157 1940 20.30777 1941 20.20233 1942 20.19973 1943 19.80717 1944 20.38696 1945 20.25326 1946 19.87021 1947 20.10624 1948 20.16717 1949 19.9762 1950 19.9495 1951 19.8693 1952 19.94103 1953 19.8344 1954 19.85546 1955 19.92559 1956 19.56086 1957 20.05878 1958 20.29486 1959 20.31079 1960 19.95747 1961 20.13461 1962 20.02131 1963 20.12156 1964 19.87288 1965 20.1846 1966 20.20451 1967 19.87006 1968 19.95379 1969 20.40858 1970 20.27722 1971 19.83824 1972 20.29696 1973 20.31497 1974 19.84183 1975 19.96552 1976 19.73089 1977 20.17786 1978 19.98341 1979 20.19729 1980 20.20056 1981 19.98539 1982 20.23017 1983 20.29599 1984 20.07477 1985 20.07888 1986 20.26229 1987 20.58401 1988 20.3868 1989 20.03639 1990 20.20498 1991 20.46869 1992 20.33603 1993 20.46631 1994 20.28377 1995 20.73241 1996 20.4509 1997 20.61819 1998 21.00812 1999 20.48336 2000 20.30699 2001 20.56523 2002 20.67353 2003 20.64243 2004 20.47049 2005 20.93473 2006 20.63209 2007 20.27468 2008 20.53754 2009 20.75372 2010 20.61774 2011 20.53226 2012 20.50895 2013 20.72398 2014 20.74841 2015 21.06683 2016 20.8883 2017 21.06791 2018 20.54848