Table 3 Relative validity

In general terms, a study of ‘relative validity’ is one that compares the performance of two or more imperfect instruments, for example, food frequency questionnaire (FFQ) relative to other self-reported instruments, such as 24-h dietary recalls and food records [104]. The evaluation of a dietary instrument can therefore involve both the assessment of its measurement error structure and its correlation with the truth (i.e. its ‘relative validity’).

Often researchers will aim to assess the ‘relative validity’ of a new dietary instrument, such as a FFQ, by comparing its results with those obtained with a more accurate measure of food or nutrient intake. This can be in the context of the development of a new instrument, to test whether it provides improvement over currently used instruments, or for the use of an existing instrument in a different population from the one in which it has been developed. The development of any given FFQ is based on the dietary intake of a defined population during a specific period in time, and when these instruments are to be used in other populations, it is important to evaluate whether the instrument gives the same results when repeated on several occasions (the ‘reproducibility’) as well as its ‘relative validity’ in the new target population [11].