From a detailed mathematical study of influenza in 39 of our largest cities, done chiefly by the means of multiple correlation, with the hope of being able to explain the differences in the epidemic curves of weekly mortality in the various cities, Pearl concludes as follows: “The general conclusion to which we come from an examination of the correlation data assembled to this point is that these four general demographic factors, density of population, geographical position, age distribution of population, and rate of recent growth in population, have practically nothing to do, either severally or collectively, with bringing about those differences between the several cities in respect to explosiveness of the outbreak of epidemic mortality in which we are interested. Significantly causal or differentiating factors must be sought elsewhere.” CHART II. [Illustration: Death rates from all causes by weeks in certain large cities of the United States during the winter of 1918–19. (_Pearl._) ] CHART III. [Illustration: Death rates from all causes by weeks in certain large cities of the United States during the winter of 1918–19. (_Pearl._) ] CHART IV. [Illustration: Death rates from all causes by weeks in certain large cities of the United States during the winter of 1918–19. (_Pearl._) ] CHART V. [Illustration: Death rates from all causes by weeks in certain large cities of the United States during the winter of 1918–19. (_Pearl._) ] CHART VI. [Illustration: Death rates from all causes by weeks in certain large cities of the United States during the winter of 1918–19. (_Pearl._) ] CHART VII. [Illustration: Death rates from all causes by weeks in certain large cities of the United States during the winter of 1918–19. (_Pearl._) ] Concerning geographical position, he did find some slight relationship with linear distance from the city of Boston, where the epidemic was supposed first to have begun in this country: “This result means that the greater the linear distance of a city from Boston the less explosive did the outbreak of epidemic mortality in that city tend to be. This is in accord with the general epidemiological rule that the force of an epidemic tends to diminish as it spreads from its primary or initial focus. It must be noted, however, that the correlation coefficient in this case is not large. It is barely past the value where it may safely be regarded as statistically significant. This fact may probably be taken to mean that influenza does not follow the epidemiological law referred to with anything like such precision as do some other epidemic diseases, notably poliomyelitis.”