For the purpose of this report, we collectively refer to “calibration studies” to indicate studies that either (i) aim to assess systematic error by comparing a dietary assessment instrument with “true exposure” (or “gold standard” reference instrument) or with a known superior dietary instrument which may also be prone to its own measurement error as the reference instrument (an “alloyed gold standard”); (ii) aim to assess random error by taking repeat measurements using the same dietary instrument. Calibration studies can be “internal” if they are performed on a subsample of the main study, or “external” otherwise . Calibration studies that use repeat measurements are common because under the classical measurement error model the error prone measurements of dietary intake are described as unbiased measures of ‘true’ exposure. This is due to the fact that under the classical measurement error model (i.e. errors in repeat measurements are uncorrelated) the average over a large number of repeated measurements would provide a good estimate of the ‘true’ exposure .