Life Tables.— Life tables art prepared for the purpose of estimating the probabilities of life and death. They are of importance in all interests in whidt it is necessary to. forecast the future of large numbers of people for com mercial or other purposes. They are indis pensable for life insurance companies since they enable them to fix their insurance rates upon a basis which has been 'found to be suf ficiently accurate for business success, during a long series of years, and with constant obser vation and study.
In constructing a fife table the faetors'wHich are necessary are the population, the age and sex distribution, the returns of death for a year or a series of years, arranged according to sex and age at death, and certain mathematical formulm.
These tables, together with certain necessary corrections, perinn the determination of the following propositions: (1) The mean lifetime. This is the sum of the ages at death, in a given community, di vided by the number of deaths.
It becomes reliable only when the con stitution of the population is uniform and the observations are carried over a long period.
(2) The probable duration of life. This is the age at which one-half of any number of children who have been born will have died, so that they will have an equal chance of dying before and after that age. It is also called the
equation of life.
(3) The mean duration of life. ThiS is the probable duration of life from the date of birth.
(4) The expectation of fife, or the mean after-lifetime. This is the average 'of years which an individual of any given age will live, as shown by 'a life table.
Bibliography.— Harrington and Richard son, (A Manual of Practical Hygiene) (1911); Census Bulletin 109, Mortality Statistics, 1910; Newsholme, 'Elements of Vital Statistics) (1899) ; Farr, (English Life Table) (1864); Longstaff, (Studies in Statistics) (1891) ; Fisher, (The Mathematical Theory of Probabili ties) (1915) ; Bowley, (The Measurement of Social Phenomena) (1915) ; Elderton and Fip pard, (Construction of Mortality and Sickness Tables) (1914) ; Yule, (Introduction. to the' Theory of Statistics) (1912); Henderson,, (Mortality Laws and Statistics) (1915) ; Wil bur, C. F., (Federal Registration Service of the Umted States, its Development Problems, and Defects) (VVashington' Government Printing. Office 1916).