PAPP101 - S09: Migration and population distribution

Migration from a set of rates (cont.)

Bases for net rates

The difficulty of choosing a base population to calculate an appropriate net migration rate is problematic. A logical consistent population base would be the entire population of the Country. The UN (1970) has stated that this is not very useful "for a given migration interval, such rates will bear the same relationship to one another as do the amounts of net migration. They therefore do not have any analytical value for a single migration interval. They are, however, potentially useful for studying time trends for individual areas" (p.41).

Rates based on the total population of a country are not commonly used. Instead one uses different denominators for different purposes. For example the base  could be the population of the particular area being studied (UN 1970). Alterantively,  an in-migration rate could be based on the population of agiven area at the end of the period, including all the in-migrants who arrived to the region in the time span. The rate would then be seen as the proportion of the population of the area at the end of the period (as in  Quiz 10, page 3).

Thus, using the data from the previous exercise on page 5, the population base would be the population of São Paulo State in 2000. The ratio would be 991,313/36,969,476, or 2.68%. The rate using the population as risk of moving (population of the remainder of Brazil in 1995) was 991,313/137,877.017 or 0.746%. The rate is 3.6 times greater using the population of São Paolo rather than the remainder of Brazil as the denominator. Thus care should be taken when calculating and interpreting net migration rates.

There are other bases that can be used for these rates depending on the purpose of the task or comparison. For example, UN (1970) states that in some instances it is preferable to use the same base for all three rates - in-migration, out-migration, and net migration and an average base could be used. For example, the average population at risk could be estimated as : ½ (36,969,476 + 36,713,680), i.e. the average of the 1995 and 2000 populations. In this case, the net migration rate is ( 991,313 - 735,517 ) / 36,841,578 = 0,69%).

In this case,the rates may be interpreted as measures of the relative importance of migration in respect to the region's population (UN, 1970, p.42).