By comparing the several experiences of the communities examined, people have been able to obtain information as to the relative importance of institutional segregation and of the other factors of the death-rate from tuberculosis. This chapter examines the records of a large number of communities exhibiting the respective variations of the several factors affecting the death-rate from tuberculosis side by side with the variations of this death-rate. The attempt to find a royal road to truth and to express it as a whole by suppressing essential parts, leads too often to indolent work and slovenly thought; and this in the public health service is not to be tolerated. In practice it is rare for two groups of phenomena to be free from disturbing influences; and the correlation-coefficient measures therefore for practical purposes the influence of one group of phenomena on the other to such extent as it predominates over or is assisted by the other influences in operation.