This is the first method we have discussed in which an element of analysis occurs and as it name implies, the method enables natural and migratory changes to be handled separately.
By examining past data on net migration rates and by attempting to relate these to economic conditions, particularly to the demand for employment in the study area, it will be possible to adopt varying assumptions about the patterns of future migration. These might simply be ‘high’ and ‘low’ e.g. +5000 persons per annum and +1000 respectively; they may be expressed as different programmes e.g. +1000 per annum in the first 5 years, +2000 per annum in the following three years and +3000 per annum in the final 12 years (of a 20-year projection) would be programme 1. Other programmes would be developed which reflected as far as possible different assumptions about the size and timing of new job opportunities, housing land capacities, the output of the construction industry, the expansion of utilities and so on.
Next a set of programmes of future natural change would be developed either by subjective projection of past maximum and minimum rates or by ‘stepping down’ fro projections produced nationally or regionally. Of course, in some cases, projected natural changes might be available for the study area itself.
The essence of the method is to begin with the starting data population, add the net migratory element to produce the next figures to which is then added the natural change thus completing one cycle of the projection. The cycle may be for 1 year, 2 years, 5 years or other convenient period. The process is then repeated until the end of the projection period. Separate exercises are carried out for each set of natural change and migration assumptions.
Several points must be noted. Whilst this method is likely to be more accurate then the crude and simple methods so far discussed there are nevertheless a number of imperfections which limit its usefulness. First, the method uses total populations; age/sex structure is not accounted for. This means that changes in birth and death rates (the elements of natural change) which might result from changing age/sex structure cannot be seen and acted upon. Nor, of course, does the planner have the direct benefit of knowing this information for future times when estimating school-age population numbers of women of working age, etc. Whilst recognizing its great shortcoming, the migration and natural increase method does reveal the possible sequence and the main elements of change far better than the methods previously outlined. At the same time it is scarcely more time-consuming or expensive.