Managing Outliers in Adolescent Food Frequency Questionnaire Data

Abstract

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.

Trial registration: ClinicalTrials.gov NCT01350531.

Keywords: adolescent; energy intake; food frequency questionnaire; outlier.