Equity in humanitarian logistics: a systematic framework for metrics, functions, modeling, and optimization

Equity in humanitarian logistics (HL) has received considerable attention. However, its implementation remains highly context-specific, resulting in heterogeneous, nonstandard, and ad hoc practices. This study systematically reviews two decades of HL research on equity and proposes a structured four-step framework comprising identifying equity metrics, constructing equity functions, formulating equity models, and selecting optimization techniques to help researchers and practitioners design and advance equity strategies tailored to specific needs. We develop a multidimensional taxonomy of equity metrics and propose strategies for switching and aligning metrics across the disaster lifecycle. We provide selection guidance for equity functions from two complementary perspectives—mathematical properties and application scenarios—and set out procedures for multi-objective settings that prevent equity dilution and address high-dimensional Pareto challenges. We compare three equity-related optimization approaches and derive choice rules with actionable thresholds based on problem scale and data availability. We also deliver scenario-specific checklists for earthquakes, floods, droughts, and pandemics within the four-step framework. Finally, we outline future directions from five equity-related perspectives to enhance both the theoretical understanding and practical application of equitable resource allocation in complex humanitarian contexts.