The increase or decrease of a country's labor force during a given period of time can be factored into the following components: A. Loss by death of labor force members. B. Net gain or loss by immigration and emigration of labor force members. C. Gain by entry into the labor force of individuals from the economically inactive population. D. Loss by retirement from the labor force into economically inactive status (including involuntary withdrawal on account of disability or for other reasons, as well as voluntary retirement). Likewise the change in number of workers attached to a given occupation or industry group of the labor force can be factored into the same four components, plus the fifth component: E. Net gain or loss by occupational or industrial mobility, i. e. transfers of labor force members from one occupation or industry to another. It is useful to subdivide components C and D as follows: C1 and D1. Labor force entries and retirements which would correspond to the maintenance of unchanging age-specific rates of entry and retirement (in the labor force as a whole and in given occupation or industry categories). C2 and D2. Entries and retirements due to changes during the period in the age-specific entry and retirement rates. The sum of components A, C1, and D1 can be considered as a measure of "natural increase" in the labor force as a whole or a given occupation or industry. This is the increase which would result from natural increase of the population and associated changes in its age structure without migration and without occupational or industrial mobility. Components B, C2, D2 and E are media through which the natural increase is modified under the influence of supply and demand factors.