It may be noted that the stable population model is a deterministic model and results in a stationary population in which the number of persons at any age (x) does not change with time. Further, suppose the individuals are born at rate where is a constant and t is time. Then the size of population at time t, or

This population grows or declines at rate , and its age distribution, i.e., proportion surviving to age x is proportional to . Using the property of stable populations that age distributions of two stable populations never cross each other census growth rate and proportion of population up to a certain age (normally 35 years) were used to estimate birth and death rates for those populations which lacked reliable and complete data on them. Development of model populations, showing probability of surviving from birth to different ages at different levels of life expectancy, for different regions of the world helped the demographers working on the populations of developing countries immensely (UN, 1983). Subsequently, the stable population theory was modified to accommodate for changes in mortality or fertility or both. Nath and Kalita (1989) developed a two-sex quasi-stable population in the presence of immigration.