1. Correcting for measurement error in nutritional epidemiology Ruth Keogh MRC Biostatistics Unit MRC Centre for Nutritional Epidemiology in Cancer Prevention and Survival IPH Showcase, 14 February 2011

2. Measuring dietary intake Food frequency questionnaires (FFQ) Diet diaries/24 hour recalls Biomarkers There is no gold standard measurement UK dietary cohort consortium EPIC-Norfolk + 5 other cohorts Case-control studies nested within cohorts 4-7 day diaries, FFQs Colorectal cancer, breast cancer, prostate cancer What is the association between long term or usual dietary intake and disease risk?

3. Food frequency questionnaires (FFQ)

4. Diet diaries

6. Choice of instrument FFQs Designed to measure long term intake Inexpensive  used for large populations Subject to substantial measurement error Diet diaries Measure actual intake Very expensive to process More highly correlated with objective biomarkers of intake Still subject to error UK dietary cohort consortium One of only a small number of studies using diaries/24 hr recalls as the main instrument Interested in correcting for error in dietary diary measurements

7. Effects of measurement error 1.Biased associations – usually attenuated 2.Loss of power to detect associations 3.Can hide nonlinear association shapes

8. Effects of measurement error 1.Biased associations – usually attenuated 2.Loss of power to detect associations 3.Can hide nonlinear association shapes

9. Effects of measurement error 1.Biased associations – usually attenuated 2.Loss of power to detect associations 3.Can hide nonlinear association shapes

10. Correcting for measurement error True diet-disease association Estimating  when we can’t observe T i Linear regression calibration model

11. Fitting the model We need to understand the structure of the error in diet diary measurements R i Random error in diary measurements: Suppose we have another diary measurement for some people In fact we can show that

12. Fibre intake and colorectal cancer OR per 6g/day increase 95% CI Using FFQ0.900.75-1.08 Using diet diary0.840.71-1.00 Corrected for measurement error in diary 0.770.62-0.97 Dahm CC, Keogh RH et al. Dietary Fiber and Colorectal Cancer Risk: A Nested Case–Control Study Using Food Diaries. JNCI 2010

13. However… We do not believe that diet diaries are subject only to random error Biomarker studies suggest error in diaries depends on true intake errors in replicate diaries are not independent We cannot estimate the extra parameters using repeat measurements to correct for error we need unbiased measurements, e.g. biomarker biomarkers not available for most nutrients Different assumptions about the type of error in R i can give very different corrected estimates

14. Challenges Measurement error can have severe effects on observed diet- disease associations What we assume about the form of the error can have strong effects on ‘corrected’ estimated associations Biomarkers can help us to understand the structure of the measurement error Also… We often want to adjust for other dietary variables, such as total energy intake All dietary variables in the model are measured with error…