Post on 15-Jan-2016
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
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?
Food frequency questionnaires (FFQ)
Diet diaries
Diet diaries
Choice of instrumentFFQs• 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
Effects of measurement error1. Biased associations – usually attenuated
2. Loss of power to detect associations
3. Can hide nonlinear association shapes
Effects of measurement error1. Biased associations – usually attenuated
2. Loss of power to detect associations
3. Can hide nonlinear association shapes
Effects of measurement error1. Biased associations – usually attenuated
2. Loss of power to detect associations
3. Can hide nonlinear association shapes
Correcting for measurement error
i
i
T = True average daily intake
R =Diet diary measurement
True diet-disease association
i i ilogit {Pr(D =1|T )} =α+βT
Estimating when we can’t observe Ti
i i i ilogit {Pr(D =1|R )} =α+βE(T |R )
Linear regression calibration model
i 0 1 i iT = λ + λ R +e
Fitting the model
We need to understand the structure of the error in diet diary measurements Ri
i 0 1 i iT = λ + λ R +e
Random error in diary measurements:
i i iR = T +Suppose we have another diary measurement for some people
i1 i i1
i2 i i2
R = T +
R = T +In fact we can show that
i2 0 1 i1 iR = λ + λ R +e
Fibre intake and colorectal cancer
OR per 6g/day increase
95% CI
Using FFQ 0.90 0.75-1.08
Using diet diary 0.84 0.71-1.00
Corrected for measurement error in diary
0.77 0.62-0.97
Dahm CC, Keogh RH et al. Dietary Fiber and Colorectal Cancer Risk: A Nested Case–Control Study Using Food Diaries. JNCI 2010
However…We do not believe that diet diaries are subject only to random error
i i iRandom error R = T +
Biomarker studies suggest
• error in diaries depends on true intake• errors in replicate diaries are not independent
i i iSystematic error R = α+βT +
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 Ri can give very different corrected estimates
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…