High-quality data generated in routine management information systems is crucial for the success of long-term public health programs since inadequate data quality can result in erroneous decisions, leading to wasted time and resources and underperformance of such programs. Data quality is measured in multiple dimensions and there can be important differences in how data are generated and compiled depending on the local context. As data quality is context-dependent, a quality assurance method is needed that fits the local context with adequate validity and reliability. A method and data quality measurement tool were developed using desk reviews, key informant interviews, and a modified Delphi technique. The method was implemented in a case study to measure the data quality at the patient-provider interface of the school dental service in Sri Lanka, giving attention to the diverse characteristics of the data generation process. The method involved an item checklist. The relative importance of item structure was considered during item selection. Weighted averages were calculated to determine the data quality at the patient-provider interface. The proposed method to develop a tool and assess data quality in public health programs could be applied as a guide in similar contexts.