{ "background": "The manufacturing sector in Nigeria faces persistent challenges in optimising production systems for cost-effectiveness. Existing evaluations often rely on retrospective data or theoretical models, lacking rigorous, real-world evidence on the comparative performance of different operational methodologies.", "purpose and objectives": "This case study aimed to methodologically evaluate and compare the cost-effectiveness of two distinct manufacturing systems—a lean cellular system versus a traditional batch-processing system—within the operational context of Nigerian industrial plants.", "methodology": "A randomised field trial was conducted across multiple manufacturing plants. Plants were randomly assigned to implement either the lean cellular (treatment) or traditional batch (control) system. Cost-effectiveness was measured via a composite metric integrating unit production cost and throughput. The primary analysis used a linear mixed-effects model: $y{ij} = \\beta0 + \\beta1 T{ij} + uj + \\epsilon{ij}$, where $y{ij}$ is the cost-effectiveness score for plant $j$ in period $i$, $T{ij}$ is the treatment indicator, $uj$ is a plant-level random effect, and $\\epsilon{ij}$ is the error term. Robust standard errors were calculated.", "findings": "The lean cellular system demonstrated superior cost-effectiveness, with an estimated mean improvement of 18.7% (95% CI: 12.3% to 25.1%) over the traditional batch system. This gain was primarily driven by a 22% reduction in work-in-progress inventory and a 15% increase in labour productivity. The treatment effect was statistically significant (p < 0.01).", "conclusion": "The randomised field trial provides robust evidence that lean cellular manufacturing systems can be significantly more cost-effective than traditional batch systems in the studied industrial setting. The methodological approach demonstrates the feasibility of rigorous experimental design in real-world engineering management.", "recommendations": "Manufacturing practitioners should prioritise the adoption of lean cellular configurations where product variety and volumes permit. Policymakers and