mine because data that permit the calculation are scarce. Using data collected in the Beltsville One Year Dietary Survey (Mertz and Kelsay, 1984), Tarasuk and Beaton (1992) investigated intake patterns for several nutrients and produced estimates of, among other parameters, the day-to-day variance in intakes for those nutrients. Other estimates have been developed from research databases and from large survey data sets with replicate observations (e.g., the Continuing Survey of Food Intakes by Individuals [CSFII]). Table B-2, Table B-3, Table B-4 through Table B-5 present pooled estimates of the day-to-day variance in intakes based on an analysis of the 1994–1996 CSFII data. Since a reliable estimate of the day-to-day variability in intakes for a specific individual is not typically available, the pooled estimates in Table B-2, Table B-3, Table B-4 through Table B-5 should be used. This introduces other uncertainties, however.
The method described to compare an individual's observed intake to the EAR for the purpose of drawing conclusions about the usual intake of the individual cannot be implemented in all cases. Even when the appropriate calculations are carried out, incorrect conclusions may result if estimates of the SD of daily intake and the SD of requirements are incorrect. These two situations are discussed below.
The value of the ratio D/SDD critically depends on the SD of daily intake for the individual. It is recommended that the estimate obtained from CSFII (see Table B-2, Table B-3, Table B-4 through Table B-5) be used for all individuals, even though it has been argued that the day-to-day variability in intakes is typically heterogeneous across individuals. Several researchers, including Tarasuk and Beaton (1992), have argued that day-to-day variability in intakes varies across individuals (see also Nusser et al., 1996); therefore a pooled variance estimate as suggested here might not be the optimal strategy. In theory, if many days of intake data Yj were available for an individual, the within-individual variance in intakes could be computed in the standard manner: