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Validity coefficients (VCs) are computed from
the following formulas derived using theory from factor analysis:
VCQT =
√((rQR *
rQM)/rRM);
VCRT =
√((rQR *
rRM)/rQM);
VCMT =
√((rQM *
rRM)/rQR).
VCQT, VCRT and VCMT are the validity coefficients between the true intake and the FFQ, recalls and the biomarker, respectively. As pointed out before, VCs measure the correlation between true exposure (T) and the measurements from each surrogate method. Validity coefficients are expected to lie between 0 and 1 but in cases of small sample size or high variation in the pair-wise correlations or when one of the correlations is negative, VCs can lie outside 0 and I boundary or may even be inestimable. This situation is referred to as a Heywood case (see references). |