Reconstructing subdistrict-level population denominators in Yemen after six years of armed conflict and forced displacement

Abstract

Introduction: Yemen has experienced widespread insecurity since 2014, resulting in large-scale internal displacement. In the absence of reliable vital events registration, we tried to reconstruct the evolution of Yemen's population between June 2014 and September 2021, at subdistrict (administrative level 3) resolution, while accounting for growth and internal migration.

Methods: We reconstructed subdistrict-month populations starting from June 2014 WorldPop gridded estimates, as a function of assumed birth and death rates, estimated changes in population density, net internal displacement to and from the subdistrict and assumed overlap between internal displacement and WorldPop trends. Available displacement data from the Displacement Tracking Matrix (DTM) project were subjected to extensive cleaning and imputation to resolve missingness, including through machine learning models informed by predictors such as insecurity. We also modelled the evolution of displaced groups before and after assessment points. To represent parameter uncertainty, we complemented the main analysis with sensitivity scenarios.

Results: We estimated that Yemen's population rose from about 26.3 M to 31.1 M during the seven-year analysis period, with considerable pattern differences at sub-national level. We found that some 10 to 14 M Yemenis may have been internally displaced during 2015-2016, about five times United Nations estimates. By contrast, we estimated that the internally displaced population had declined to 1-2 M by September 2021.

Conclusions: This analysis illustrates approaches to analysing the dynamics of displacement, and the application of different models and data streams to supplement incomplete ground observations. Our findings are subject to limitations related to data quality, model inaccuracy and omission of migration outside Yemen. We recommend adaptations to the DTM project to enable more robust estimation.

Keywords: Armed conflict; Forced displacement; Humanitarian; Internally displaced; Population; Yemen.

Fig. 1

Reductions in subdistrict missingness achieved after each successive data management step. Only eligible IDP records are included in the denominator.

Fig. 2

Illustration of four hypothetical scenarios (A to D) in which a group of IDPs arrives to a subdistrict from another subdistrict. In scenario A, nearly all IDPs remain in the subdistrict of refuge throughout the period of interest. In scenario B, only a fraction are left by the second assessment round. In scenario C, all IDPs have left the subdistrict (either returned to their subdistrict of origin, or moved elsewhere) by the second assessment, and in scenario D IDPs have left even before the first assessment, thereby potentially being missed altogether by the displacement tracking system.

Fig. 3

Predictions of a generalised additive mixed growth model of IDP population as a function of time. Panel A shows the model's predicted percent change in IDP group size by month since displacement: the thick line is the main analysis prediction, and the shaded area indicates the 95%CI. Predictions using reasonable-high and -low adjustments for non-monotonic series are also shown. Panel B shows the model's predictions (blue dots and 95% confidence bands) and observed total prevalent number of IDPs assessed (orange squares) during any given month after displacement, for the main analysis only.

Fig. 4

Countrywide average values of crude birth rate and crude death rate assumed, by scenario. The grey dotted line shows secular trends based on UN projections; the latter are represented by squares and centred at the mid-point of their period of reference. The shaded area indicates the crisis period.

Fig. 5

Estimated population of Yemen over time, by scenario.

Fig. 6

Estimated number of IDPs in Yemen over time, by scenario. Squares with labels (in millions or M) indicate year-end estimates from the Internal Displacement Monitoring Centre (Internal Displacement Monitoring Centre, 2021).

Fig. 7

Estimated percentage of IDPs amongst the entire population, by district, as of September 2021. Thick boundaries and text labels denote governorates; light boundaries denote districts.