{ "background": "Public health surveillance systems are critical for early detection and response to disease outbreaks, yet their operational reliability in low-resource settings is often assumed rather than rigorously measured. There is a paucity of empirical evidence on the consistency of data capture and reporting within these systems.", "purpose and objectives": "This study aimed to conduct a methodological assessment of the reliability of routine public health surveillance systems in Uganda, using a randomised field trial to quantify measurement error and system consistency.", "methodology": "We conducted a randomised, controlled field trial across 120 health facilities. Synthetic case reports for notifiable diseases were introduced into the surveillance workflow at randomly assigned facilities. System reliability was measured as the proportion of introduced cases correctly captured and reported through each tier of the system. Data were analysed using a multilevel logistic regression model: $\\logit(p{ij}) = \\beta0 + \\beta1 X{ij} + uj$, where $p{ij}$ is the probability of report capture for case $i$ in facility $j$, $X{ij}$ represents intervention covariates, and $uj$ are facility-level random effects.", "findings": "The overall reliability, defined as complete and accurate reporting from facility to national level, was 68.2% (95% CI: 62.1% to 73.8%). Reliability varied significantly by disease type and facility level, with central hospitals outperforming lower-level health centres. The odds of correct reporting were 2.3 times higher (95% CI: 1.7 to 3.2) for facilities with dedicated surveillance officers.", "conclusion": "The study provides the first experimental evidence that surveillance system reliability in this setting is suboptimal and heterogeneous, indicating substantial measurement error in routine data.", "recommendations": "Surveillance strengthening programmes should prioritise supportive supervision and dedicated human resources at lower-level facilities. Methodological protocols for routine reliability audits should be integrated into national surveillance guidelines.", "key words": "