Mixed
In longitudinal dietary intervention or crossover trials, the ratio of period-to-period standard deviation to day-to-day standard deviation (RP) determines the trade-off between the number of subjects and the number of measurement days required to achieve statistical precision.
When designing a diet study, do not assume a fixed number of days works for everyone. Calculate the 'RP ratio' (period variation vs. daily variation) for your specific population. If subjects are eating freely (high daily variation), you need more measurement days to get accurate averages, whereas controlled feeding allows fewer days. The biggest precision gains come from the first few days of measurement.
The ratio of period-to-period and day-to-day standard deviations controls the trade-off between the number of days and the number of subjects, and this remained relatively stable across studies and energy and macronutrient intake variables.
Why this rating
Based on analysis of three distinct datasets (two exercise cohorts, one Olestra crossover) using linear mixed models.
Source
Number of days, number of subjects, and sources of variation in longitudinal intervention or crossover feeding trials with multiple days of measurement
Gary K. Grunwald et al. · British Journal Of Nutrition · 2003
DOI 10.1079/bjn2003989
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