# year value 1901 20.131 1902 20.00494 1903 19.83207 1904 19.84611 1905 19.54037 1906 19.89276 1907 20.04909 1908 19.81743 1909 19.99381 1910 19.83518 1911 19.78226 1912 19.97553 1913 19.82705 1914 20.0283 1915 20.06527 1916 19.92844 1917 19.91868 1918 19.59545 1919 20.27039 1920 19.82683 1921 19.92366 1922 20.28728 1923 19.99352 1924 20.33331 1925 19.41527 1926 20.21589 1927 20.0316 1928 20.29317 1929 20.03468 1930 19.76037 1931 20.44157 1932 20.21025 1933 20.20432 1934 19.93114 1935 20.24694 1936 20.31419 1937 20.26407 1938 20.51853 1939 20.38148 1940 20.3552 1941 20.68836 1942 20.4182 1943 20.00551 1944 20.21734 1945 19.88418 1946 20.22748 1947 20.40122 1948 20.28089 1949 19.85414 1950 19.85546 1951 19.9244 1952 20.18206 1953 20.42451 1954 20.18597 1955 20.19495 1956 20.00265 1957 19.67151 1958 20.39008 1959 20.21251 1960 20.30533 1961 20.06519 1962 19.76959 1963 20.41371 1964 19.88508 1965 19.99041 1966 20.31081 1967 19.84696 1968 19.55997 1969 20.49951 1970 20.49829 1971 19.89784 1972 19.76253 1973 20.42475 1974 19.68109 1975 19.73678 1976 19.74514 1977 20.14572 1978 20.27676 1979 20.63765 1980 20.35873 1981 20.00179 1982 20.23094 1983 19.73102 1984 19.8726 1985 20.24632 1986 20.19087 1987 20.35283 1988 20.59526 1989 19.58952 1990 20.31027 1991 20.73987 1992 19.69382 1993 20.08584 1994 20.42265 1995 20.10905 1996 20.67694 1997 20.35432 1998 20.88074 1999 20.89922 2000 20.34367 2001 20.5125 2002 20.77295 2003 20.99264 2004 20.79947 2005 20.84549 2006 21.18497 2007 20.56884 2008 20.28755 2009 21.11209 2010 21.47178 2011 20.78333 2012 20.41991 2013 20.98821 2014 20.52909 2015 20.65617 2016 20.83855 2017 21.07622 2018 20.88101