1905 26.7
1906 25.8
1907 24.9
1908 23.9
1909 22.9
1910 21.8
1911 20.6
1912 19.5
1913 18.3
1914 17.1
1915 15.9
1916 14.7
1917 13.5
1918 12.4
1919 11.3
1920 10.2
1921  9.1
1922  8.2
1923  7.3
1924  6.4
1925  5.7
1926  5.1
1927  4.5
1928  4.1
1929  3.7
1930  3.4
1931  3.2
1932  3.1
1933  3.0
1934  3.0
1935  3.0
1936  3.1
1937  3.2
1938  3.3
1939  3.4
1940  3.5
1941  3.6
1942  3.7
1943  3.8
1944  3.9
1945  3.9
1946  3.9
1947  3.8
1948  3.7
1949  3.5
1950  3.3
1951  3.1
1952  2.9
1953  2.6
1954  2.3
1955  2.0
1956  1.7
1957  1.5
1958  1.2
1959  1.0
1960  0.8
1961  0.7
1962  0.6
1963  0.6
