{ "background": "Industrial process-control systems are critical for operational efficiency, yet there is a paucity of rigorous field evidence from the region comparing the efficacy of different methodological approaches for yield optimisation.", "purpose and objectives": "This study aimed to empirically evaluate and compare the yield improvement performance of three distinct process-control system methodologies—model predictive control, statistical process control, and a rule-based expert system—within operational industrial plants.", "methodology": "A multi-site randomised field trial was conducted across 27 plants in the chemical and manufacturing sectors. Plants were randomly assigned to one of the three intervention methodologies or a control group. Yield was measured as mass output per unit of raw material input over a standardised period. The primary analysis used a linear mixed-effects model: $Y{ij} = \\mu + \\alphai + \\beta X{ij} + \\epsilon{ij}$, where $Y_{ij}$ is the yield for plant $j$ in sector $i$, with robust standard errors clustered at the plant level.", "findings": "The model predictive control system generated a statistically significant mean yield increase of 7.3% (95% CI: 5.1% to 9.5%) compared to the control group, outperforming both the statistical process control (3.1% increase) and rule-based expert system (1.8% increase) methodologies.", "conclusion": "The trial demonstrates that advanced model-based control methodologies can deliver substantial yield gains in real-world industrial settings, whereas simpler systems offer more modest improvements.", "recommendations": "Plant managers should prioritise investment in model predictive control systems where feasible, given its superior performance. Further research should investigate the cost-benefit analysis of implementation across different industrial scales.", "key words": "process control, randomised controlled trial, yield optimisation, industrial engineering, manufacturing", "contribution statement": "This paper provides the first comparative, experimental evidence from a randomised field trial on the effectiveness of different process-control methodologies for yield improvement in an