Journal Article

An Approach to Estimate Between- and Within-Group Correlation Coefficients in Multicenter Studies: Plasma Carotenoids as Biomarkers of Intake of Fruits and Vegetables

Pietro Ferrari, Wael K. Al-Delaimy, Nadia Slimani, Hendriek C. Boshuizen, Andrew Roddam, Philippos Orfanos, Guri Skeie, Miguel Rodríguez-Barranco, Anne Thiebaut, Gerd Johansson, Domenico Palli, Heiner Boeing, Kim Overvad and Elio Riboli

in American Journal of Epidemiology

Published on behalf of Johns Hopkins Bloomberg School of Public Health

Volume 162, issue 6, pages 591-598
Published in print September 2005 | ISSN: 0002-9262
Published online September 2005 | e-ISSN: 1476-6256 | DOI: http://dx.doi.org/10.1093/aje/kwi242
An Approach to Estimate Between- and Within-Group Correlation Coefficients in Multicenter Studies: Plasma Carotenoids as Biomarkers of Intake of Fruits and Vegetables

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In a multicenter study, the overall correlation between two variables can be broken down into a within- and a between-group correlation reflecting associations at the individual and aggregate levels, respectively. A random-effects model is used to estimate variance components of nutrition-related variables and the within- and between-group correlation coefficients. Using data from the European Prospective Investigation into Cancer and Nutrition (EPIC), the authors analyzed the association between levels of three plasma carotenoids (alpha-carotene, beta-cryptoxanthin, and lycopene) and dietary intake of three items (total fruits, carrots, and tomatoes), assessed through dietary questionnaire and single 24-hour dietary recall measurements, in a cross-sectional study involving 3,089 subjects from nine European countries. Intraclass correlation coefficients were 0.178 for alpha-carotene, 0.216 for beta-cryptoxanthin, and 0.299 for lycopene. The between-group correlation coefficients were higher than the within-group coefficients for all three carotenoids. For beta-cryptoxanthin and fruit intake, the between-group versus the within-group correlations were 0.78 and 0.26 for the dietary questionnaire and 0.85 and 0.19 for the 24-hour dietary recall. Results indicate that variability of exposure is driven mainly by the individual compared with the aggregate variation and that biomarker levels perform fairly accurately in discriminating population-level consumption of fruits and vegetables.

Keywords: carotenoids; diet; epidemiologic methods; multicenter studies; random-effects model; DQ, dietary questionnaire; EPIC, European Prospective Investigation into Cancer and Nutrition; ICC, intraclass correlation coefficient; 24-HDR, 24-hour dietary recall

Journal Article.  5305 words. 

Subjects: Public Health and Epidemiology

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