OBJECTIVES: Patient-reported outcomes (PROs) collected as endpoints in clinical trials often require patients to report symptom severity, frequency, or the impact of symptoms on quality of life. It is generally accepted that variability in PROs should be reduced to ensure data quality, however, assessment variability due to the subject’s interpretation of measurement parameters may be overlooked. METHODS: We collected data from 485 individuals who were asked to respond to the following scenario, “If you were participating in a clinical trial that asked you to report how many days you had a headache in a week, and you had a headache from 8:00 pm Sunday night to 8:00 am Monday morning, does it count as 1 or 2 days with a headache?”. In addition, demographic data, as well as whether patients have ever participated in clinical trials, were also collected. RESULTS: Among the total participants, 434 people responded to this question. A strong majority of responders (76.5%, n = 332) answered the question incorrectly. Only 23.5% (102 patients) answered the question correctly (“