1900 38.600002
 1901 33.9
 1902 33.9
 1903 33.5
 1904 37.100002
 1905 33.8
 1906 34.9
 1907 34.4
 1908 32.5
 1909 32.5
 1910 29.7
 1911 37.7
 1912 34.4
 1913 31.2
 1914 33.7
 1915 33.
 1916 31.800001
 1917 34.4
 1918 36.4
 1919 35.2
 1920 31.800001
 1921 37.100002
 1922 34.8
 1923 36.100002
 1924 32.9
 1925 31.6
 1926 32.
 1927 30.2
 1928 36.3
 1929 34.8
 1930 34.2
 1931 32.
 1932 35.7
 1933 36.2
 1934 33.600002
 1935 33.100002
 1936 32.4
 1937 34.4
 1938 33.5
 1939 32.9
 1940 32.100002
 1941 35.3
 1942 32.5
 1943 36.600002
 1944 34.8
 1945 36.8
 1946 36.9
 1947 40.4
 1948 33.7
 1949 33.600002
 1950 33.600002
 1951 30.4
 1952 38.
 1953 34.600002
 1954 34.3
 1955 33.
 1956 31.1
 1957 36.
 1958 31.1
 1959 36.4
 1960 30.5
 1961 34.4
 1962 32.100002
 1963 31.
 1964 35.2
 1965 30.5
 1966 32.7
 1967 33.
 1968 34.
 1969 32.8
 1970 31.2
 1971 31.6
 1972 30.1
 1973 32.7
 1974 34.
 1975 35.7
 1976 35.4
 1977 29.
 1978 30.1
 1979 32.100002
 1980 31.300001
 1981 31.9
 1982 32.8
 1983 33.4
 1984 34.8
 1985 32.
 1986 34.9
 1987 33.
 1988 31.1
 1989 33.
 1990 36.600002
 1991 34.5
 1992 35.9
 1993 31.6
 1994 35.3
 1995 36.4
 1996 34.100002
 1997 34.4
 1998 37.3
 1999 33.
 2000 33.
 2001 35.2
 2002 34.8
 2003 39.5
 2004 33.5
 2005 34.3
 2006 37.4
 2007 33.3