Epidemiological research guides health service delivery by informing and transforming clinical guidelines. Reciprocally, how clinicians document information from patients or their loved ones also determines the quality of data epidemiologists use in research. Additionally, some patients’ health needs and some communities’ health interests are not well captured in observational studies and clinical trials, especially when patient-subjects are lost to follow-up. Attrition of patient-subjects who are uninsured, underinsured, experience transportation or food insecurity, work multiple jobs, or live with complex chronic illnesses or disabilities, for example, creates data capture gaps, which can result in an evidence base that is incomplete and clinical guidelines that are inapt, inequitable, or simply do not apply to a full range of patients. This theme issue investigates these and other complexities of epidemiological data sourcing and the ethics of practice guideline formation.