When two measures are correlated, measurement error can lower the correlation coefficient below the level it would have reached if the measures had been free from measurement error. A de-attenuated correlation coefficient can be computed to correct for attenuation due to within-person variation if repeat measurements are available on the reference method. If for example, the dietary instrument was a FFQ and the reference instrument were multiple food diaries the de-attenuated correlation (ρ), under the assumption of a classical measurement error model, could be obtained by the formula: ρ = r √[1 + (wpv/bpv)*n] |

Where r is the observed correlation; wpv is the within-person variance of the reference method; bpv is the between-person variance of the reference method; and n is the number of repeat measurements of the reference method [18]. Often variation due to daily energy intake is removed by adjusting for total energy using the residual method [106] prior to accounting for within-person variation in order to produce energy-adjusted de-attenuated correlations. |