It is desirable that investigations into the mortality experienced by assured lives and more especially by annuitants should be carried out by continuous methods which involve a minimum of time and labour in order that the nature and extent of changes in the mortality from time to time may be observed. The Census Method* meets these requirements, and this method is examined “with the view of establishing how far it can in “itself or with practicable modifications cope with certain questions “arising (1) out of the rapid increase in the rate of mortality during the “period in which selection is in operation and (2) from the disturbance “caused by the heaping-up of business towards the end of each financial “year.” No suitable experience on which to test the method was available, and it was decided to construct hypothetical data in which known rates of mortality prevail in order that some measure of the probable numerical magnitude of the errors in practice might be obtained.