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 deattenuated correlation coefficient can be computed to correct for attenuation due to withinperson 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 deattenuated correlation (ρ), under the assumption of a classical measurement error model, could be obtained by the formula:
ρ = r √[1 + (wpv/bpv)*n]
