One of the most common models of population growth used by demographers is the so-called Leslie model, developed in the 1940 s. This model describes the growth of the female portion of a human or animal population. In this model the females are divided into age classes of equal duration. To be specific , suppose that the maximum age attained by any female in the population is L years (or some other time unit) and we divide the population into n age classes . Then each class is L/n years in duration. We label the age classes according to table 1.