Misclassification bias occurs when individuals, values, or data are assigned to an inappropriate category. The erroneous data categorization can obscure or affect results. In this study, researchers used pharmacy records to categorize patients. Therefore, over-the-counter medications may not be included in the data, and patients who were taking OTC NSAIDs may have been categorized as taking warfarin only. Medicaid often pays for OTC medications, so perhaps the results are more accurate in the Medicaid group than the Medicare group.
(Source: https://www.healthknowledge.org.uk/e-learning/epidemiology/practitioners/errors-epidemiological-measurements)