Methodological Diagnostics of Municipal Water Systems in Rwanda: A Systematic Review of the Difference-in-Differences Model for Yield Optimisation

{ "background": "Municipal water systems are critical for agricultural productivity and rural development. In Rwanda, optimising the yield of these systems is a policy priority, yet rigorous methodological evaluations of impact assessment models, particularly quasi-experimental designs, are lacking in the literature.", "purpose and objectives": "This systematic review aims to critically evaluate the application and methodological rigour of the difference-in-differences (DiD) model for measuring yield improvements in Rwandan municipal water systems within the agricultural sector.", "methodology": "A systematic search of multiple academic databases was conducted following a pre-registered protocol. Eligible studies were those employing a DiD design to assess water system interventions on agricultural yield. Studies were appraised for methodological quality, focusing on model specification, identification assumptions, and robustness checks. The canonical two-way fixed effects model is $Y{it} = \\alpha + \\beta (Treati \\times Postt) + \\gammai + \\deltat + \\epsilon{it}$.", "findings": "The review identified a limited corpus of studies. A dominant theme was the frequent failure to test the parallel trends assumption, with fewer than 30% of studies employing placebo tests or event-study frameworks for validation. Where reported, treatment effect estimates were often presented without confidence intervals or robust standard errors, limiting inferential certainty.", "conclusion": "The application of the DiD model in this specific context is methodologically nascent. While the design is theoretically well-suited, common shortcomings in empirical practice undermine the reliability of reported yield optimisation impacts.", "recommendations": "Future research must prioritise rigorous pre-intervention trend analysis and comprehensive robustness checks. Policymakers should require such diagnostic evidence when evaluating programme efficacy. Capacity building in advanced quasi-experimental methods for local researchers is essential.", "key words": "difference-in-differences, water resources, agricultural productivity, impact evaluation, quasi-experimental design, public utilities", "contribution statement": "This review provides the first dedicated methodological audit of DiD applications in