Purpose: To develop a test of a measure of health numeracy that is based on an empirically derived framework, cross-culturally equivalent for Hispanic and Non-Hispanic populations, and uses Item Response Theory scaling methods.
Method: A cross-cultural approach was used in the development of the Numeracy Understanding in Medicine Instrument (NUMi). Qualitative methods used to generate the item bank (n=110) included focus groups, convening of an expert panel, and cognitive interviews. Participants (n=1000) were recruited from community and clinical populations to test the items. A 2-Parameter IRT model was used for analysis of the first 500 respondents. Participants also responded to the Test of Functional Health Literacy in Adults-Short Form (S-TOFHLA) and a cognitive reasoning and aptitude test (Wonderlic). A sample (n=200) of participants were also administered the Lipkus numeracy scale and a mathematic achievement test (WRAT-M).
Result: Of the first 500 participants 50% were White and 40% were Black. Over 30% were Hispanic and 40% had a high school level education. Based upon the IRT parameters of difficulty (range of -3.0 t0 3.0) and discrimination (range of 0 to 3.0), 20 items were chosen to form the NUMi. The most difficult items in each domain assessed the following skills: 1) Determining how many 500 mg pills add up to 3 grams (number sense), 2) interpreting a risk of 2/1000 in a pictograph (tables and graphs), 3) interpreting a relative risk reduction (probability), and 4) interpreting the meaning of a p-value that is < 0.05 (statistics). The Test Information Function peaked at a difficulty level of -1.0 indicted that the test is most discriminating for people with lower than average health numeracy. Test performance was positively associated with cognitive reasoning (0.79) and print health literacy (0.54).
Conclusion: The NUMi is a 20 item paper and pencil test that measures an empirically based construct of health numeracy. In future work, a computerized adaptive test will be developed in which items are selected based upon the ability of the respondent leading to a shorter test that will reduce respondent burden and allow for oral or print administration. The NUMi will provide a feasible and valid assessment of health numeracy skills for use in both research and clinical settings.
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