Differential item functioning analysis of the 2003-04 NHANES physical activity questionnaire.

Using differential item functioning (DIF) analyses, this study examined whether there were any DF items in the National Health and Nutrition Examination Survey (NHANES) physical activity (PA) questionnaire. A subset of adult data from the 2003-04 NHANES study (n = 3,083) was used. PA items related to respondents' occupational, transportation, domestic, leisure-time, strength-related, and sedentary activities were analyzed for DIF using Mantel-Haenszel, SIBTEST, and analysis of variance procedures. Some items were identified as DF items, with the majority of those items favoring advantaged social groups. For example, items about domestic activity and moderate-intensity leisure-time activity were DF items favoring non-Hispanic Whites, and persons with higher levels of education (e.g., some college or more) or income (e.g, annual family income [AFI] > $34,999). The vigorous activity item was a DIF item favoring younger adults (ages 20-59 years) and persons with higher levels of education or income. Occupational PA presented DF favoring Hispanics and persons with lower levels of education (e.g., high school or less), and the transportation-related PA item presented DF favoring persons with lower incomes (e.g., AFI $34,999). These findings highlight the importance of conducting DF analysis in PA survey construction and emphasize the need to introduce DIF concepts and methods to PA researchers.