Crosssectional Studies

Cross-sectional studies examine variables at a single point in time. Without the possibility of establishing temporal precedence, typically cross-sectional studies cannot establish causality. Three sorts of analysis are available here: (1) tests of the strength of association between variables; (2) significance of differences between groups in the quantity of a variable; and (3) difference in the rate of occurrence of an event (commonly an outcome of interest). In nutritional gerontology, it is important not only to estimate the number of subjects required to test the study hypothesis, but also to decide upon the optimum type of dietary assessment. Within an individual old person there may be considerable variation over time, possibly reflecting the vagaries of social opportunities to share a meal. Longer periods of dietary intake may be required to take account of this yet; in some old people this is prone to faulty recall. To allow for this, it is usual to rely on old people for relatively brief periods of recall and to compensate for this by increasing the size of the sample. It is, however, not an easy task to decide on the best combination of observation period and sample size. The choice is influenced by the purpose served by the data. If it is necessary to relate dietary intake of each individual to some other characteristic of the same individual, then the most accurate measures of dietary intake are required (usually, seven days or more of weighed intake less food waste). However, this labor-intensive method is not required if it is more important to identify subjects at the extremes of a food-intake distribution or to make comparisons between mean values of each group. There are many comprehensive accounts of cross-sectional study designs that are well summarized by Cade (1997).

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