# year value 1901 19.63327 1902 19.508 1903 18.91966 1904 18.811 1905 19.00339 1906 19.54094 1907 19.14493 1908 19.33699 1909 19.12583 1910 18.83817 1911 18.95261 1912 19.39529 1913 19.78696 1914 19.74616 1915 19.4069 1916 19.5473 1917 19.29145 1918 19.51637 1919 19.74251 1920 19.61135 1921 19.2816 1922 19.04945 1923 19.42773 1924 18.78034 1925 19.03074 1926 19.6109 1927 19.25682 1928 18.66842 1929 19.44554 1930 19.4228 1931 19.18314 1932 19.71329 1933 19.02744 1934 19.1968 1935 18.97635 1936 19.00356 1937 19.0547 1938 19.01196 1939 19.76902 1940 19.02301 1941 18.88815 1942 19.42871 1943 19.74095 1944 19.45442 1945 19.51385 1946 19.21588 1947 19.31766 1948 19.06551 1949 19.17341 1950 19.33992 1951 19.25065 1952 19.65085 1953 19.36813 1954 19.30025 1955 19.72277 1956 18.8951 1957 19.2922 1958 19.34991 1959 19.20141 1960 19.74242 1961 19.61253 1962 19.46187 1963 19.60216 1964 19.15792 1965 19.30309 1966 19.08294 1967 19.04691 1968 19.87417 1969 19.49499 1970 19.70564 1971 18.92019 1972 19.33271 1973 19.88171 1974 19.24362 1975 19.34653 1976 19.39619 1977 19.48194 1978 19.8003 1979 20.15062 1980 19.80901 1981 20.09379 1982 19.8136 1983 20.10732 1984 19.77456 1985 19.49963 1986 19.41559 1987 19.73433 1988 19.64529 1989 20.15251 1990 19.97837 1991 19.91907 1992 19.13437 1993 19.52839 1994 19.38262 1995 19.92808 1996 19.48292 1997 19.74338 1998 19.62326 1999 20.21856 2000 19.57255 2001 20.32549 2002 19.27161 2003 20.0404 2004 19.99964 2005 19.94149 2006 20.04578 2007 20.02459 2008 20.0884 2009 20.34698 2010 20.00356 2011 20.07683 2012 20.20078 2013 20.37158 2014 20.45867 2015 20.11106 2016 20.59249 2017 20.53527 2018 20.73553