The Versatility of the Life Table

cases of illness, marriages, items of industrial equipment, or any other entity, for present purposes reference will be made principally to human lives. Consider a countable aggregate or cohort of newly born lives, each of which is identified by one or more common characteristics of a specified nature. Some or all of the lives may also have characteristics other than those specified to define the group, but these are of no concern at this point. In the course of time, the group will gradually lose individual lives, the process of attrition continuing until the last life has dropped out. It may be assumed, further, that a means is on hand either to enumerate the lives remaining in the group at the end of stated time intervals or to count the number of lives lost to the group during each time interval. These time intervals need not necessarily be equal, but in dealing with human lives, the year is a convenient unit. After the last life is gone, it is a simple matter to compute the total years of life lived by the entire group and so the average number of years lived by the individual lives since the group was formed. In the same way, at any point of time in the history of the group, a computation may be made of the total years of life remaining to the individuals surviving at that time, and of the average number of years remaining to the individual lives. It is also possible to compute the ratio of the number of lives lost within a year to the number in the group at the beginning of that year or to the average number in the group during that year. Such ratios are termed "rates" by the vital statistician and the actuary.* Since age is described by elapsed time, the foregoing averages and rates may be