The Mouse Infectivity Titration of Influenza Virus

A study has been made to establish the statistical significance of results obtained in mouse infectivity titrations of influenza virus. Five titrations, each composed of five replicas, were carried out and 50 per cent end points were calculated for each titration. Three criteria for evaluating the end points were employed, namely, the presence or absence of pulmonary lesions, the occurrence of death, and a weighted composite taking into account both the extent of lung consolidation and the occurrence of death. Standard deviations of the distribution of end points obtained by each method were computed, and from these data levels of probabilities for significance in the differences between end points were determined. It was found that the chances are 19 out of 20 that differences of 0.99, 0.77, and 0.73 logarithmic units, respectively, for the lesion, the death, and the weighted end points are significant.