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American Journal of Epidemiology Advance Access originally published online on August 10, 2005
American Journal of Epidemiology 2005 162(6):591-598; doi:10.1093/aje/kwi242
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American Journal of Epidemiology Copyright © 2005 by the Johns Hopkins Bloomberg School of Public Health All rights reserved

PRACTICE OF EPIDEMIOLOGY

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

Pietro Ferrari1, Wael K. Al-Delaimy1, Nadia Slimani1, Hendriek C. Boshuizen2, Andrew Roddam3, Philippos Orfanos4, Guri Skeie5, Miguel Rodríguez-Barranco6, Anne Thiebaut7, Gerd Johansson8, Domenico Palli9, Heiner Boeing10, Kim Overvad11 and Elio Riboli1

1 Unit of Nutrition and Cancer, International Agency for Research on Cancer, Lyon, France
2 National Institute of Public Health and the Environment (RIVM), Bilthoven, The Netherlands
3 Cancer Research UK, Radcliffe Infirmary, University of Oxford, Oxford, United Kingdom
4 University of Athens Medical School, Athens, Greece
5 Institute of Community Medicine, University of Tromsø, Tromsø, Norway
6 Epidemiology Department, Murcia Health Council, Murcia, Spain
7 INSERM, XR 251, Institut Gustave Roussy, Villejuif, France
8 Department of Nutritional Research, University of Umeå, Umeå, Sweden
9 Molecular and Nutritional Epidemiology Unit, Centro per lo Studio e la Prevenzione Oncologica, Scientific Institute of Tuscany, Florence, Italy
10 German Institute of Human Nutrition, Potsdam-Rehbrücke, Germany
11 Department of Clinical Epidemiology, Aalborg Hospital, Aarhus University Hospital, Aalborg, Denmark

Correspondence to Dr. Pietro Ferrari, Unit of Nutrition and Cancer, International Agency for Research on Cancer, 150 cours Albert-Thomas, 69372 Lyon Cedex 08, France (e-mail: Ferrari{at}iarc.fr).

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.

carotenoids; diet; epidemiologic methods; multicenter studies; random-effects model

Abbreviations: DQ, dietary questionnaire; EPIC, European Prospective Investigation into Cancer and Nutrition; ICC, intraclass correlation coefficient; 24-HDR, 24-hour dietary recall


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