Background: Health numeracy can be defined as the ability to understand and apply information conveyed with numbers, tables and graphs, probabilities, and statistics to effectively communicate with health care providers, take care of one’s health, and participate in medical decisions. Objective: To develop the Numeracy Understanding in Medicine Instrument (NUMi) using item response theory scaling methods. Design: A 20-item test was formed drawing from an item bank of numeracy questions. Items were calibrated using responses from 1000 participants and a 2-parameter item response theory model. Construct validity was assessed by comparing scores on the NUMi to established measures of print and numeric health literacy, mathematic achievement, and cognitive aptitude. Participants: Community and clinical populations in the Milwaukee and Chicago metropolitan areas. Results: Twenty-nine percent of the 1000 respondents were Hispanic, 24% were non-Hispanic white, and 42% were non-Hispanic black. Forty-one percent had no more than a high school education. The mean score on the NUMi was 13.2 (s = 4.6) with a Cronbach α of 0.86. Difficulty and discrimination item response theory parameters of the 20 items ranged from −1.70 to 1.45 and 0.39 to 1.98, respectively. Performance on the NUMi was strongly correlated with the Wide Range Achievement Test–Arithmetic (0.73, P < 0.001), the Lipkus Expanded Numeracy Scale (0.69, P < 0.001), the Medical Data Interpretation Test (0.75, P < 0.001), and the Wonderlic Cognitive Ability Test (0.82, P < 0.001). Performance was moderately correlated to the Short Test of Functional Health Literacy (0.43, P < 0.001). Limitations: The NUMi was found to be most discriminating among respondents with a lower-than-average level of health numeracy. Conclusions: The NUMi can be applied in research and clinical settings as a robust measure of the health numeracy construct.
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