ESTIMATION OF THE NET MIGRATION BY COMPARING TWO SUCCESSIVE CENSUSES. Michel POULAIN GéDAP UCL Belgium. The following methodology concerns the estimation of the net international migration. By definition this is the difference between immigration and emigration figures.
By definition this is the difference between immigration and emigration figures.
Net migration is equal to zero for internal migrations.
If data on international immigrations is available and enough reliable, then the international emigration figure may be estimated by difference between the international immigration figure and the net migration estimation EMI = IMMI – (IMMI-EMI).
Population by sex and year of birth at national level at the time of first census.
Population by sex and year of birth at national level at the time of second census.
Number of births and deaths by sex and year of birth for the intercensal period (the date of occurrence of death is not needed for this calculation).
The date of census is not usually 1 January (or 31 December), but recalculated figures for official population estimates often have to be provided for these particular dates each year.
We will present a concrete example on Estonia where the two last censuses where carried out on 12ve January 1989 and 31st March 2000.
The used data are recalculated figures for 1st January 1989 and 2000.
The reliability of Census data must be examined carefully before starting the estimation of the net migration
In fact, census errors may have a substantial impact on estimated net migration figures and therefore under-coverage or double-counts by age and sex are useful information for carrying out our estimations.
First, it is essential to identify which specific populations that were not concerned by either or both censuses such as army, refugees, asylum seekers, short-term immigrants etc. More generally there will be some problems if census enumeration rules vary from one census to the next or if the census rules do not match those followed in vital registration as regards the population at risk.
Let us take two concrete examples:
The second census considers refugees but the first did not, while no immigration has been registered for these refugees. In this situation there are two possibilities to make everything consistent: either not to consider refugees in the second census or to include them ex post in immigration figures.
The first census included some army personnel but the second does not, while these persons were not recorded in emigration statistics.
So far we may consider under-coverage arising quasi-randomly or relating to a non-covered specific population.
We have also to face another possible source of error: untrue responses to census questions and more specifically to census questions relate to age or data of birth. This may result in age exaggeration by the oldest or some attraction towards specific ages.
For specific age attraction some smoothing correction methods may be used, but none will be appropriate to correct age exaggeration.
Information is also needed about the reliability and coverage of all recorded demographic events (births, deaths, international immigration and international emigration).
Birth and death registration are both usually considered as broadly reliable, and limited errors in the associated statistical data collection process will usually have no impact on the estimation of the net migration.
However, we have to take care of two specific points:
Do vital statistics data cover all years during the intercensal period, including the few months between exact census dates and the time of official annual population figure release?
Do the vital statistics include all specific groups of population, following the same rules as census enumeration?
It is essential that both censuses and vital statistics relate to the same population. If all exclude the same specific group(s) there will be no negative impact on the method. However if a specific group is missing in one data source, it is probably easier to eliminate the same group from the other data source than to try to adjust the data source from which it is excluded.
The basic equation to explicit the population change is the following :
Population at second census T 2 =
Population at first census T 1 + births (T1, T2) – deaths (T1, T2)
+ immigrations (T1, T2) – emigrations (T1, T2)
= immigrations (T1, T2) – emigrations (T1, T2)
Net migration (T1, T2) =
Population at second census T 2 - Population at first census T 1
- births (T1, T2) + deaths (T1, T2)
For various reasons linked to over-coverage or under-coverage of some specific populations the real population stock at a given census would be accompanied by a confidence interval
P observed (T1) – Over-estimation (T1)
< P real (T1)
< P observed (T1) + Under-estimation (T1)
P observed (T2) – Over-estimation (T2)
< P real (T2)
< P observed (T2) + Under-estimation (T2)
P real (T1) = P observed (T1) + Error term (T1)
P real (T2) = P observed (T2) + Error term (T2)
where error terms may be positive or negative depending if we are facing under-coverage (positive error) or over-coverage (negative error).
Net migration (T1, T2) = P real (T2) - P real (T1)
- births (T1, T2) + deaths (T1, T2)
Net migration (T1, T2) = P observed (T2) + Error term (T2) - P observed (T1) - Error term (T1) - births (T1, T2) + deaths (T1, T2)
This equation allows estimating net migration for a given generation. As the age of a given generation will be different in the two censuses and if error is age dependant we cannot not agree that compensation will occur between the two error terms.
Let’s assume at that stage that the error terms may be neglected and that births and deaths figures by generation are well known
The two last censuses have been organised respectively on 12ve January 1989 and
31st March 2000.
The Estonian Statistical Office provided an appropriate estimation of the population on 1st January 1989 and 2000 based on both census enumerations and all changes in the population occurring between census and estimation times.
Deaths (T1, T2) = k. [P (T1) + (Net migration)/2]
Net migration (T1,T2) = [P(T2) - P(T1) (1-k)
- births (T1, T2)]
/(1/(1 – k/2))
Net migration is the difference between agregate numbers of immigrations and emigrations by age and sex but people immigrating are never similar to those emigrating as they are reacting to clearly different stimuli…
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