Managing Outliers in Adolescent Food Frequency Questionnaire Data

Objective: The goal of this study was to explore the impact of 5 decision rules for removing outliers from adolescent food frequency questionnaire (FFQ) data. Design: This secondary analysis used baseline and 3-month data from a weight loss intervention clinical trial. Participants: African American adolescents (n = 181) were recruited from outpatient clinics and community health fairs. Variables Measured: Data collected included self-reported FFQ and mediators of weight (food addiction, depressive symptoms, and relative reinforcing value of food), caregiver-reported executive functioning, and objectively measured weight status (percentage overweight). Analysis: Descriptive statistics examined patterns in study variables at baseline and follow-up. Correlational analyses explored the relationships between FFQ data and key study variables at baseline and follow-up. Results: Compared with not removing outliers, using decision rules reduced the number of cases and restricted the range of data. The magnitude of baseline FFQ-mediator relationships was attenuated under all decision rules but varied (increasing, decreasing, and reversing direction) at follow-up. Decision rule use increased the magnitude of change in FFQ estimated energy intake and significantly strengthened its relationship with weight change under 2 fixed range decision rules. Conclusions and Implications: Results suggest careful evaluation of outliers and testing and reporting the effects of different outlier decision rules through sensitivity analyses.