DIETARY ASSESSMENT METHODOLOGY Scandinavian Journal of NutritionlNaringsforskning Vo142:60-62, 1998

Dietary investigations- what are the effects of invalid selection procedures and measurement errors? By Lars Berglund In observational studies where average levels or percentiles in a population distribution of nutritional components are to be assessed it is essential that the participating subjects are representative of the population to which the results are to be generalised. The optimum way of achieving this is by random selection from the target population in which all members have equal probability (or at least a known probability) of being chosen and also that all selected subjects participate in the study. The effects of selection bias and non-response can be large and are in general difficult to estimate. In the presentation of results it should always be stated how selection bias and non-response could have influenced the results. In this paper a general selection procedure and certain problems with it is presented. A method for correction of nonresponse bias is also presented. The effect of measurement errors on correlation coefficients between nutritional components or between a nutritional component and other variables is discussed