Duplicate policies in mortality data

The question of the effect of duplicate policies on the variance of the number of deaths in a mortality experience based on policies (not lives) has been discussed in two recent papers (Seal [1947] and Beard and Perks [1949]). The second of these papers shows that this variance depends on the sampling process envisaged and gives formulae for the variance under four such processes. It is the purpose of this paper to consider some practical aspects of the treatment of duplicates in a mortality experience.