{ "background": "Process-control systems are critical for industrial and infrastructure sectors, yet systematic reliability diagnostics in developing contexts are underdeveloped. The absence of standardised field-evaluation methodologies hinders targeted maintenance and capacity planning.", "purpose and objectives": "This working paper presents a methodological evaluation of a novel randomised field trial (RFT) framework designed to quantify and diagnose the reliability of process-control systems. The primary objective is to test the RFT's efficacy in generating robust, actionable reliability metrics under operational field conditions.", "methodology": "A randomised field trial was implemented across multiple industrial sites. System performance was continuously monitored, with interventions randomised across treatment and control units. Reliability was modelled using a Weibull survival function, $R(t) = \\exp^{-(t/\\eta)^\\beta}$, where $\\eta$ is the scale parameter and $\\beta$ the shape parameter. Inference was based on robust standard errors clustered by site.", "findings": "The RFT methodology proved feasible and generated high-fidelity diagnostic data. A key finding was that systems in the intervention arm showed a 22% lower hazard rate of critical failure modes compared to control systems (95% CI: 15% to 29%). Diagnostic data highlighted sensor calibration drift as a predominant, previously under-recognised failure initiator.", "conclusion": "The randomised field trial provides a rigorous, transferable methodological framework for reliability assessment. It moves beyond retrospective failure analysis to enable proactive, evidence-based system management.", "recommendations": "Adopt the RFT framework for baseline reliability audits in major infrastructure projects. Integrate diagnostic findings into preventative maintenance schedules, prioritising sensor calibration protocols. Further trials should validate the method across different climatic zones and system vintages.", "key words": "reliability engineering, randomised controlled trial, process control, maintenance, field study, survival analysis", "contribution statement": "This paper provides a novel methodological framework for field-based reliability diagnostics, demonstrating its application and value through a large-scale