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American Journal of Epidemiology Advance Access originally published online on March 8, 2006
American Journal of Epidemiology 2006 163(8):770-778; doi:10.1093/aje/kwj082
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American Journal of Epidemiology Copyright © 2006 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.

Practice of Epidemiology

Validity and Systematic Error in Measuring Carotenoid Consumption with Dietary Self-report Instruments

Loki Natarajan1, Shirley W. Flatt2, Xiaoying Sun1, Anthony C. Gamst1, Jacqueline M. Major2, Cheryl L. Rock2, Wael Al-Delaimy2, Cynthia A. Thomson3, Vicky A. Newman2, John P. Pierce2 for the Women's Healthy Eating and Living Study Group

1 Division of Biostatistics, University of California, La Jolla, CA
2 Cancer Center, University of California, La Jolla, CA
3 University of Arizona, Tucson, AZ

Correspondence to Dr. Loki Natarajan, Cancer Center, University of California, San Diego, 3855 Health Sciences Drive, La Jolla, CA 92093-0901 (e-mail: loki{at}math.ucsd.edu).

Vegetables and fruits are rich in carotenoids, a group of compounds thought to protect against cancer. Studies of diet-disease associations need valid and reliable instruments for measuring dietary intake. The authors present a measurement error model to estimate the validity (defined as correlation between self-reported intake and "true" intake), systematic error, and reliability of two self-report dietary assessment methods. Carotenoid exposure is measured by repeated 24-hour recalls, a food frequency questionnaire (FFQ), and a plasma marker. The model is applied to 1,013 participants assigned between 1995 and 2000 to the nonintervention arm of the Women's Healthy Eating and Living Study, a randomized trial assessing the impact of a low-fat, high-vegetable/fruit/fiber diet on preventing new breast cancer events. Diagnostics including graphs are used to assess the goodness of fit. The validity of the instruments was 0.44 for the 24-hour recalls and 0.39 for the FFQ. Systematic error accounted for over 22% and 50% of measurement error variance for the 24-hour recalls and FFQ, respectively. The use of either self-report method alone in diet-disease studies could lead to substantial bias and error. Multiple methods of dietary assessment may provide more accurate estimates of true dietary intake.

bias (epidemiology); carotenoids; diet; models, statistical; nutrition assessment; reproducibility of results

Abbreviations: FFQ, food frequency questionnaire; WHEL, Women's Healthy Eating and Living


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