{ "background": "Industrial machinery fleets in developing economies face significant operational and safety risks, yet robust methodological frameworks for quantifying the impact of targeted interventions are lacking. Existing evaluations often rely on before-and-after comparisons, which fail to account for underlying trends and external factors.", "purpose and objectives": "This short report aims to demonstrate the application of a quasi-experimental difference-in-differences (DiD) model to rigorously evaluate a structured maintenance and operator training programme implemented within Tanzanian industrial fleets. The objective is to provide a methodological blueprint for isolating the causal effect of such interventions on machinery-related incident rates.", "methodology": "A retrospective cohort study was designed, analysing incident data from treatment and control fleets over a multi-period panel. The core DiD model is specified as $Y{it} = \\beta0 + \\beta1 \\text{Treat}i + \\beta2 \\text{Post}t + \\delta (\\text{Treat}i \\cdot \\text{Post}t) + \\epsilon{it}$, where $Y{it}$ is the incident rate for fleet $i$ in period $t$. Inference is based on cluster-robust standard errors at the fleet level.", "findings": "The intervention led to a statistically significant reduction in the mean incident rate. The DiD estimator, $\\delta$, was -0.18 incidents per 10,000 operational hours (95% CI: -0.27 to -0.09), representing a 22% reduction relative to the control group's trend. Sensitivity checks supported the model's parallel trends assumption.", "conclusion": "The difference-in-differences approach provides a more credible estimation of risk reduction compared to simpler methods, effectively controlling for confounding temporal effects. This confirms the efficacy of the integrated maintenance and training programme.", "recommendations": "Adopt quasi-experimental designs, particularly DiD models, for evaluating engineering safety interventions in industrial settings. Fleet managers should implement integrated technical and human-factor