On the basis of a paper by K. Wendel, and an I. M. C. O. (Intergovernmental Maritime Consulative Organization) report, new I. M. C. O. rules of November 1973 are briefly explained. These rules are essentially based on statistical analyses and are tested experimentally on models to determine A, the Attained Subdivision Index, which is the keynote of the new regulation and is an index, although purely approximate, of the probability of the survival of a ship. The purpose of the paper is to indicate the significance of the various factors entering into the expression for A.