The following explanation of certain technical terms and symbols that are used in this book is given as an assistance to non-actuarial readers. For a fuller account of the functions and notation, reference can be made to the text-books for actuaries, Compound Interest and Annuities-Certain by D. W. A. Donald and Life and Other Contingencies by P. F. Hooker and L. H. Longley-Cook. They are published by the Cambridge University Press.

When an investigation is made into the mortality experienced among lives assured, the number of persons entering at each age, and the numbers passing out of observation at each age owing to (1) death, (2) withdrawal, by the policies lapsing, being surrendered or terminating from some other cause, are recorded. The exposed to risk of death at age 25 (E25) means the number who had the chance of dying between ages 25 and 26, and were on the average at risk for the whole year.

The number who die between ages 25 and 26, divided by the exposed to risk at age 25, gives the probability of dying in a year at age 25 (q25). Consequently, deaths are tabulated according to age last birthday when the exposed to risk are tabulated according to the exact age or an approximation thereto; if, as is often necessary, the investigation is made according to the number of years an insurance has been in force, the exposed to risk are tabulated according to an exact duration and deaths according to the integral number of years in force (curtate duration).