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In epidemiological studies, there is always the ‘true’ but sometimes unknown (latent) value of the exposure e.g. true lycopene intake. Interestingly, it can be shown using theory from factor analysis (see references) that, if linear relationships exist between the true exposure and measurements by the three methods, then the 3 measurements can be related to the true exposure using a mathematical formula. In the next slide we show a pictorial relationship between the three methods (Q, R and M) and the true exposure (T).