{ "background": "Public health surveillance systems are critical for early detection and response to outbreaks, yet their operational reliability in resource-limited settings is often assumed rather than rigorously measured. This creates a significant evidence gap for system strengthening and investment.", "purpose and objectives": "This case study aimed to develop and apply a novel field trial methodology to quantitatively assess the reliability of a national notifiable disease surveillance system, using acute watery diarrhoea (AWD) as a tracer condition.", "methodology": "A randomised field trial was conducted across 60 health facilities in two regions. Standardised simulated case reports were introduced at randomly selected points in the reporting chain. System reliability was measured as the proportion of introduced signals successfully captured and transmitted to the central level. Performance was modelled using a generalised linear mixed model: $\\logit(P{ij}) = \\beta0 + \\beta X{ij} + ui + \\epsilon{ij}$, where $P{ij}$ is the probability of successful signal transmission for facility $j$ in district $i$, $X$ represents covariates, and $u_i$ are district-level random effects.", "findings": "The overall system reliability for complete signal transmission was 58% (95% CI: 52% to 64%). Reliability varied significantly by reporting level, with a pronounced decline at the district-to-region interface, where approximately 30% of signals were lost. The mixed-effects model indicated that facility type and staff training levels were significant predictors of successful reporting.", "conclusion": "The applied randomised trial method provided a robust, quantifiable measure of surveillance system reliability, revealing substantial operational fragility that routine performance indicators had not captured.", "recommendations": "Surveillance strengthening programmes should prioritise interventions at the district-level aggregation point and integrate routine reliability assessments using similar field trial methodologies. Investment in digital reporting tools should be coupled with enhanced training for data handlers.", "key words": "surveillance, reliability, field trial, health systems, evaluation, health information systems