Computation of the significance of the differences between adjacent qualitative groups shows that the difference in the biological efficacy of radium compared to 200 kV roentgen rays is highly significant (p < -001), and the ratio of the two (1 : 0-7) is in excellent agreement with other estimates (MacComb and Quimby, 1937; Quastler and Clark, 1945; Sugiura, 1939).
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