American Journal of Clinical Nutrition, Vol 55, 22-27, Copyright © 1992 by The American Society for Clinical Nutrition, Inc

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Statistical estimation of dietary parameters: implications of patterns in within-subject variation--a case study of sampling strategies

V Tarasuk and GH Beaton
Department of Nutritional Sciences, Faculty of Medicine, University of Toronto, Ontario, Canada.

Patterns within intraindividual variation in energy intake were described previously. Using case studies based on the same Beltsville One-Year Dietary Intake Study data set, we examined the interaction between random and nonrandom variation and the choice of sampling strategy in estimation of individuals' usual intakes over 1 y. Mean intake estimates derived from adjacent-day samples were less reliable and more likely to be biased than were those based on randomly selected days. A finite adjacent-day sample fails to encompass longer-term trends. Because adjacent-day samples underestimate true within-subject variation, by customary tests they appear more reliable. This may present an interpretational problem. Comparisons of random weekend and week-day samples confirm that failure to proportionately sample both will bias the estimation of the usual (1-y mean) intake and the within- subject variance.

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