# year value 1901 18.87594 1902 18.24919 1903 18.46359 1904 18.44311 1905 18.63581 1906 18.45196 1907 17.46754 1908 18.14272 1909 18.63727 1910 18.96316 1911 18.28493 1912 17.82758 1913 18.34656 1914 18.57633 1915 18.25319 1916 18.00682 1917 18.92328 1918 19.15523 1919 19.32978 1920 18.46952 1921 18.83681 1922 19.56722 1923 18.30405 1924 18.81829 1925 18.75724 1926 19.36423 1927 18.63359 1928 18.50099 1929 19.1728 1930 19.11481 1931 19.92573 1932 18.77913 1933 19.67108 1934 19.40932 1935 19.15055 1936 19.9683 1937 19.23366 1938 19.0488 1939 19.11965 1940 19.93167 1941 18.46496 1942 18.95813 1943 18.41962 1944 18.01246 1945 18.59113 1946 19.14169 1947 18.35795 1948 18.60687 1949 18.71968 1950 18.4491 1951 18.54681 1952 18.82627 1953 18.73371 1954 18.3953 1955 18.56246 1956 18.84037 1957 18.92378 1958 19.73345 1959 19.59107 1960 19.74279 1961 20.33414 1962 18.47544 1963 18.53327 1964 18.53838 1965 18.1513 1966 18.99792 1967 19.30659 1968 18.44774 1969 19.13312 1970 19.69237 1971 19.32271 1972 19.23855 1973 19.05109 1974 19.31609 1975 18.38316 1976 18.25045 1977 19.82418 1978 19.04371 1979 19.02003 1980 18.79968 1981 19.57735 1982 18.69666 1983 18.62027 1984 18.94998 1985 19.58977 1986 19.53678 1987 18.80921 1988 19.76094 1989 19.11463 1990 19.4176 1991 18.95578 1992 18.96406 1993 17.77357 1994 20.46542 1995 18.7903 1996 20.12112 1997 18.83188 1998 19.17825 1999 18.56801 2000 19.61877 2001 19.81246 2002 19.81073 2003 20.47215 2004 19.40957 2005 19.4202 2006 20.47135 2007 20.39806 2008 19.62354 2009 19.32842 2010 19.26997 2011 19.48844 2012 20.30105 2013 20.49101 2014 19.97706 2015 20.61117 2016 20.29483 2017 20.17067 2018 20.51968