1. Estimating Migration from Census data Issues for consideration
By Richmond Tiemoko

2. Outline Definition Issues emerging Data requirements
International Migration
Internal Migration

3. Definition
According to the United Nations recommendations, migrants consist of four categories:  - long-term immigrants (or emigrants); - short-term immigrants (or emigrants);  - residents returning after (or leaving for) a period working abroad, i.e. short-term emigrants returning (or leaving); and - nomads.
Long term Migrant: A person who moves to a country/place of usual residence other than that of his or her usual residence for a period of at least a year (12 months), so that the country/place of destination effectively becomes his or her new country/place of usual residence. From the perspective of the country of departure the person will be a long-term emigrant and from that of the country of arrival the person will be a long-term immigrant.

4. Migration data and information
Provided that the census(es) gather the appropriate information and are reasonably accurate it is technically straight forward to estimate migration from census data . It is possible to produce estimates of:
Stock of Migrants (in-migration, emigrants?)
Net immigration (i.e. immigration less emigration) of the foreign-born population (people born outside a particular country)
Internal migration between sub-national regions of a country, over the period between two censuses.
Stock of out-migrants
Migrants as a percentage of total population
Females (specific population group) among migrants
Migrants by age group
Migrants by socioeconomic characteristics.

5. Migration and Mobility
Issues emerging from census generated migration data
Mobility
Displaced and forced migration
Return migration
Citizenship and Stateless

6. Data Requirements Usual Census Questions for migration study:
Place of birth
Place of residence
Nationality/ Citizenship
Place of previous Residence
Duration of residence ( Flow)
Date of Migration

7. Data Requirements (cont…)
To estimate net immigration of foreigners*: the number of foreign-born females (males), in five-year age groups, and for an open age interval A+, at two points in time, typically two censuses
For the deaths: either a suitable model life table or the numbers of native-born females (males), in five-year age groups, and for an open age interval A+, at two points in time, typically two censuses. Failing these, the central crude death rate for the population
To estimate sub-national regional net in-migration from place of birth data: the number of females (males) by sub-national region and by sub-national region of birth, in five-year age groups, and for an open age interval A+, at two points in time, typically two censuses
The concept of Foreigner is a social and legal concept. It can affect the estimate of migration for example for people born to migrant

8. Data Requirement (cont…)
To estimate internal migration between sub-national regions from place of residence at previous census data:
The numbers of females (males) by sub-national region and by sub- national region at some prior date, typically that of the preceding census, in five-year age groups, and for an open age interval A+.
If age-specific numbers are not available, aggregated data is still useful for estimating all-age migration.

9. Data quality and preparation
Evaluate the data quality (age and sex distribution) . Difference with general population, host country population
Change in boundaries at sub-national or international levels
Net migration, by definition, underestimates the flows of migrants into and out of a region or country. Thus, for example, return migration and mobility within the period being considered will result into zero net migration.

10. Estimating Net migration using the place of birth
Step 1: Decide on survival factor
If data on the number of foreign-born people in the population are available by age group for each census then one needs to estimate the survival factors to be applied to the numbers of foreign-born in the first census to estimate the numbers surviving to the time of the second census. Use a life table
Steps 2: Estimate the number of deaths of immigrants

11. Step 3: Estimate the net number of immigrants (of foreigners)
If data are available by age group for each census then age-specific
net immigration can be estimated as follows:
 Net ​5MFx=​5NFx+n(t+n)−​∞NFx(t)+​5DFx  for x = 0, 5, … , A-5-n where
 Net ​5MFx  represents the net number of immigrants between times t
and t+n who were aged between x and x + 5 at time t.
For x > A - 5 - n;  Net ​∞MFA−n=​∞NFA(t+n)−​∞NFA−n(t)+​∞DFA−n .
The net number of immigrants of those born between times t and t+n is
estimated as follows:  Net ​MFB=​nNF0(t+n)+​DFB .
If data and/or survival factors are not available by age group then estimate
The total net number of immigrants as follows:
 Net ​∞MF0=​∞NF0(t+n)−​∞NF0(t)+​∞DF0 .

12. Example:
Table 1 Estimation of deaths of foreign-born and the net number of immigrants by age group, South Africa,
Age
2001
2006 x
5Sx
Age at 2ndcensus DF
Net M B
0- 4
8,963
12,577
391
12,968
5- 9
10,390
13,724 5
242
5,003
10-14
13,508
13,998 10 55
3,664
15-19
27,835
27,943 15
119
14,555
20-24
69,787
59,493 20
616
32,275
25-29
87,381
95,763 25
2,994
28,970
30-34
73,338
100,450 30
6,675
19,743
35-39
66,663
85,490 35
7,563
19,715
40-44
59,152
75,684 40
7,701
16,721
45-49
45,184
66,113 45
7,274
14,234
50-54
40,398
55,913 50
6,154
16,883
55-59
30,640
42,833
5,717
8,153
60-64
24,376
34,433 60
5,442
9,234
65-69
17,895
25,588 65
5,353
6,564
70-74
13,561
18,989 70
5,281
6,375
75-79
10,238
12,850 75
5,404
4,693
80-84
7,658
7,461
80+
5,118
2,341
85+
4,455
5,305
7,410
602
Total
611,423
754,608
79,509
222,693

13. Estimating Net internal Migration
Indirect Methods
Net M = (Pt+n)-Pt.- (B-D)
Survival Ratios Methods: Net M'(x) = Px+.n,t+.n-S·PX,1
The basic information required is the number of persons classified by age and sex as enumerated in each area at two successive censuses and a set of survival ratios which can be applied to the population at the first census in order to derive an estimate of the number of persons expected to survive to the second census
3.Census Survival Ratios (CSR) if you have strong reason not to use life table.

14. Estimating net internal migration between sub-national regions from place of birth data
Estimation process similar to the one of foreign born migration
Change in the number of population born in the region and living outside the region gives the estimate of net out-migration from that region
Subtracting this from the net in-migration of those born outside the region gives an estimate of the overall net in-migration into the region

15. Estimating internal migration between sub-national regions from place of residence at previous survey
Net sub-national regional migration is estimated directly from the numbers of people in each region at the time of the census who moved since the previous census by region they were in at a given prior date (e.g. at the time of the previous census).
where the superscript (’) represents numbers surviving and 5I’x and 5O’x respectively represent the number of surviving in-migrants into, and the surviving number out-migrants from, a particular region at the time of the second census who were aged between x and x+5 at the second census
The number of migrants is derived from the number of surviving
in- and out-migrants as follows:  
5Mx=(​5I′x−​5O′x+(​5I′x−​5O′x)x/​5Sx)/2 ,

16. THANK YOU
Example and Questions