{ "background": "Manufacturing systems in developing economies often operate below optimal yield levels, yet rigorous methodological frameworks for evaluating systemic improvements are scarce. This gap hinders evidence-based engineering management and resource allocation.", "purpose and objectives": "This study aims to develop and apply a quasi-experimental econometric model to quantify the causal impact of a systematic process intervention on production yield within Tanzanian manufacturing plants.", "methodology": "A difference-in-differences (DiD) model was employed, analysing panel data from treatment and control groups of plants before and after the implementation of a standardised lean manufacturing protocol. The core 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 yield. Inference is based on cluster-robust standard errors.", "findings": "The intervention generated a statistically significant positive effect. The DiD estimator ($\\delta$) was 7.3 percentage points (95% CI: 5.1, 9.5), indicating a substantial yield increase attributable to the new system. The parallel trends assumption was validated using pre-intervention data.", "conclusion": "The applied DiD model provides a robust methodological framework for isolating the effect of engineering system changes in real-world manufacturing settings, confirming the efficacy of the implemented protocol.", "recommendations": "Manufacturing engineers and plant managers should adopt quasi-experimental evaluation designs to rigorously measure process improvements. Policymakers should support the development of localised production datasets to facilitate such analyses.", "key words": "difference-in-differences, yield optimisation, manufacturing systems, process engineering, causal inference, lean production", "contribution statement": "This paper presents a novel application of the DiD econometric technique to isolate the causal impact of a manufacturing systems intervention in a