1. Monitoring international migration flows in Europe Frans Willekens
Conference of European Statistics Stakeholders (CESS)
Budapest, 21 October Session B8 – Migration: Data and Management

2. Our society cannot function without reliable statistics (Marianne Thyssen, 20 October 2016)
Statistics are indispensable for evidence-based decision making

3. Improve migration statistics
A long journey and ongoing
Issues:
Definition of migration
Usual residence
Duration threshold
Coverage
Underreporting / undercount
Accuracy of data collection
Milestone
EU Regulation 682/2007
Measurement and estimation

4. Solution: synthetic data
Census
Survey
Administrative data
Data
(Measurements)
Synthetic data
(Estimates)
Model
Other relevant information
quantitative
qualitative
Distinguish between observations (data) and ’true’ migration flows

5. Model of migration Data generating process: counting process
Count data: number of migrations
Stochastic process {N(t), t ≥ 0}, with N(t) the ’true’ migration flows
Simplest counting process: Poisson process
One parameter, but varies by migrant category
Unobserved heterogeneity: mover-stayer model
Direction of migration: Origin - destination
E(N) = λ
var(N) = λ
λ = 𝜇t

6. Model of migration How to estimate model from data?
Theory of counting processes
Number of migrations (occurrences) by origin, destination, and attributes of migrants and stayers
Exposure:
Number of persons exposed
Duration of exposure
Parameter of model: migration rate
Data are realisations of (underlying) counting process, e.g. Poisson process

7. Model of migration Use all relevant info: data and prior info
Types of prior information
Quantitative: primary data and auxiliary data
Qualitative: expert opinions
How to add prior information?
Information as probability distributions
Bayesian rule
Prior

8. Model of migration gamma distribution
gamma distribution
with shape parameter c=n+1 and scale parameter b=1/2
E(λ)=bc = (n+1)/2

9. Observed vs ‘true’ migrations
Migration model predicts ‘true’ number of migrations
Measurement model quantifies difference between observed flows and ‘true’ flows (separate for origin and destination country)
True flow = Observed flow * Correction factor

10. How to obtain prior distribution. Expert opinion or model
How to obtain prior distribution? Expert opinion or model? Example: duration criterion
UN: permanent migrant stays at least 12 months
Poisson model with duration criterion

11. How to obtain prior distribution. Expert opinion or model
How to obtain prior distribution? Expert opinion or model? Example: duration criterion
Table 1.
True migration flow (UN definition) as fraction of recorded flow. Expert judgments, Poisson model and mixture model
Duration threshold
Experts judgment
Poisson model
Mixture model
No time limit
3 months
6 months
12 months
Permanent (p))
5 years
10 years
0.51
0.61
0.81
1.00
1.64
0.79
0.84
0.89
2.61
8.67
0.64
0.77
1.80
2.98

12. How to obtain prior distribution. Expert opinion or model
How to obtain prior distribution? Expert opinion or model? Example: duration criterion
Mixture model (mover-stayer model)
𝜇 = 0.94 * * 1.8

13. Conclusion Combine data from different sources Meta data are essential
Use statistical models:
Migration model
Measurement model
Extend duration criterion to include short-term migration and circular migration
NSI should exchange migration data
Estimate emigration
Gain insight in drivers / root causes of migration: World Migration Survey

14. Thank you