Monitoring international migration flows in Europe Frans Willekens Conference of European Statistics Stakeholders (CESS) Budapest, 21 October 2016. Session B8 – Migration: Data and Management

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

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

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

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  

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

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

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

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  

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        

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  

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 * 0.01 + 0.06 * 1.8

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

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