An approach to estimate between- and within-group correlation coefficients in multicenter studies: plasma carotenoids as biomarkers of intake of fruits and vegetables

Ferrari P, Al-Delaimy WK, Slimani N, Boshuizen HC, Roddam A, Orfanos P, Skeie G, Rodríguez-Barranco M, Thiebaut A, Johansson G, Palli D, Boeing H, Overvad K, Riboli E. An approach to estimate between- and within-group correlation coefficients in multicenter studies: plasma carotenoids as biomarkers of intake of fruits and vegetables. Am J Epidemiol. 2005; 162(6): 591–8.