Total methods of projection (cont.)

Arithmetic growth

The next simplest model is that of arithmetic or linear growth. This model assumes that a constant numeric change occurs in the size of the population in every period of the same length.

A minimum of two estimates of the population for different dates are needed to estimate the annual increment in the population and project its size at other dates. The model can be fitted to a longer series of estimates of the population by a means of a simple linear regression of population size on time.

Thus, if P(t) refers to the population at time t and P(t+n) refers to the population n years later:

where a is the constant annual increase in the population