# year value 1901 20.0157 1902 19.93908 1903 19.70867 1904 19.7325 1905 20.07744 1906 19.86898 1907 19.86527 1908 19.85683 1909 20.05141 1910 19.73088 1911 19.83573 1912 19.6578 1913 19.77396 1914 19.82735 1915 20.25661 1916 19.77096 1917 19.83638 1918 19.99244 1919 19.87538 1920 20.06547 1921 19.87448 1922 19.905 1923 19.93749 1924 19.95503 1925 19.96467 1926 20.11445 1927 20.24796 1928 19.97282 1929 19.84742 1930 19.98431 1931 20.1393 1932 19.9341 1933 20.08875 1934 20.12089 1935 20.21347 1936 19.995 1937 20.2198 1938 19.96517 1939 20.12783 1940 20.28852 1941 20.25211 1942 20.29148 1943 20.08544 1944 20.00342 1945 20.15153 1946 20.31517 1947 20.20281 1948 19.78131 1949 19.98192 1950 19.8641 1951 20.3558 1952 20.05881 1953 20.02686 1954 19.97444 1955 19.92674 1956 19.67686 1957 20.05364 1958 20.07702 1959 19.96531 1960 20.17241 1961 19.77818 1962 19.98304 1963 20.21756 1964 19.78636 1965 20.01116 1966 19.94895 1967 19.52435 1968 20.01227 1969 20.07738 1970 19.76973 1971 19.53342 1972 20.14935 1973 19.94906 1974 19.91714 1975 19.79984 1976 19.76752 1977 20.11672 1978 19.95889 1979 20.09318 1980 20.28589 1981 20.06266 1982 20.13376 1983 20.36791 1984 19.88568 1985 20.09601 1986 19.94731 1987 20.54008 1988 20.12255 1989 20.2631 1990 20.4798 1991 20.00659 1992 19.89659 1993 20.27149 1994 20.25818 1995 20.46261 1996 20.35309 1997 20.42501 1998 20.90075 1999 20.43378 2000 20.26649 2001 20.58589 2002 20.53517 2003 20.68189 2004 20.4395 2005 20.76032 2006 20.71794 2007 20.47602 2008 20.48731 2009 20.88672 2010 20.71593 2011 20.65058 2012 20.67762 2013 20.52207 2014 20.7453 2015 21.19757 2016 21.07351 2017 20.97143 2018 20.56745