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American Journal of Epidemiology Advance Access published online on August 10, 2005
American Journal of Epidemiology, 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
Received August 24, 2004
Accepted April 27, 2005

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 Ferrari 1*, Wael K. Al-Delaimy 1, Nadia Slimani 1, Hendriek C. Boshuizen 2, Andrew Roddam 3, Philippos Orfanos 4, Guri Skeie 5, Miguel Rodríguez-Barranco 6, Anne Thiebaut 7, Gerd Johansson 8, Domenico Palli 9, Heiner Boeing 10, Kim Overvad 11, and Elio Riboli 1

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

* To whom correspondence should be addressed.
Pietro Ferrari, E-mail: Ferrari{at}iarc.fr


  Abstract

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.
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