# year value 1901 19.38376 1902 18.47259 1903 18.32745 1904 20.19365 1905 20.05016 1906 18.91188 1907 18.66813 1908 19.06582 1909 18.07541 1910 18.35954 1911 20.18715 1912 18.22771 1913 17.50388 1914 18.23461 1915 18.81415 1916 18.15155 1917 19.87739 1918 18.01235 1919 18.39632 1920 18.42891 1921 19.75 1922 19.10054 1923 18.80222 1924 18.21743 1925 18.81891 1926 17.96257 1927 19.55886 1928 20.53368 1929 19.73842 1930 19.54839 1931 19.5633 1932 19.56551 1933 18.77562 1934 19.27439 1935 19.95677 1936 18.8705 1937 19.56083 1938 19.60604 1939 19.28431 1940 18.04737 1941 18.99223 1942 19.32928 1943 19.67261 1944 19.68875 1945 20.35945 1946 19.98884 1947 21.07988 1948 18.36486 1949 19.23137 1950 21.23039 1951 19.44393 1952 20.9952 1953 19.13182 1954 18.59714 1955 18.82222 1956 18.5036 1957 19.44419 1958 19.42004 1959 19.68752 1960 18.76994 1961 19.05721 1962 18.99048 1963 19.49731 1964 19.89973 1965 18.38652 1966 18.62012 1967 19.52805 1968 18.54275 1969 18.57184 1970 19.41771 1971 19.64915 1972 18.77473 1973 19.72314 1974 18.99187 1975 19.11593 1976 19.42168 1977 18.63737 1978 18.04375 1979 19.04724 1980 18.43127 1981 19.13873 1982 20.14255 1983 20.71391 1984 18.63938 1985 19.28028 1986 19.44203 1987 19.18295 1988 19.81586 1989 19.48699 1990 19.85546 1991 20.19398 1992 20.77449 1993 19.86338 1994 21.40568 1995 20.09427 1996 19.51691 1997 19.8365 1998 20.62231 1999 20.18815 2000 20.44328 2001 20.34193 2002 20.72507 2003 22.34513 2004 20.09783 2005 20.06908 2006 20.63401 2007 20.61708 2008 20.61823 2009 20.86326 2010 20.80093 2011 20.40944 2012 21.53572 2013 21.14622 2014 20.07014 2015 22.30173 2016 20.88978 2017 22.0713 2018 22.08991