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In this example we have 4 measurements:
Two from an FFQ administered twice (Q1 and Q2), an average of 7 repeated
measurements from dietary recalls (R) and the measurement from plasma, a biomarker
(M). Questions: Are the 3 methods equally valid? Is the FFQ measurement fairly reproducible? The pair-wise correlations (rounded-off to 2 dec. points) are as follows: For the FFQ and recalls: rQR = 0.37; or using Q2, rQR = 0.40; For the FFQ and biomarker: rQM = 0.24 or using Q2, rQM = 0.18. For the recalls and biomarker: rRM = 0.45. The VCs (and bootstrap confidence intervals) follow:
VCQT =
√((rQR *
rQM)/rRM)
= √((0.37 * 0.24)/0.45) = 0.44 (0.21-0.59);
VCRT =
√((rQR *
rRM)/rQM)
= √((0.37 * 0.45)/0.24) = 0.84 (0.48-1.00)*;
VCMT =
√((rQM *
rRM)/rQR)
= √((0.24 *0.45)/0.37) = 0.53 (0.35-0.90).
*Heywood
case, upper 95% CL >1 has been set to 1.00.
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