1.1. Pension schemes and other plans written on an insured group basis present some of the most fascinating, challenging and paradoxical aspects of life offices’ business in the United Kingdom.

1.2. The fascination stems from each group scheme being a unique, unpredictable and sensitive unit, covering perhaps just a handful of lives or possibly many thousands of lives. Once established it may thrive and grow indefinitely.

One of the earliest problems faced by actuaries and one which has persisted to modern times is that of designing and carrying out an investigation into a certain set of decremental rates, especially mortality rates. Many investigations have been conducted with a view to the derivation of a table of mortality rates, some of them of crucial importance to the British life insurance industry. Such tables as the OM, the A1924/29 and the A1949/52 tables have been so widely known and used that mention of them is scarcely necessary. However, despite the importance of such investigations, little detailed attention has been given to methods for determining a suitable term of the experience. The OM table was derived from data covering a term of 30 years, A1924/29 from a 6-year term and A1949/52 from a 4-year term. Were these terms reasonable or not?