Chapter 6 Reproducibility: duplicate measurements of the same individual in the same situation and time frame. Validity: comparison of questionnaire data to results from a different tool

Approaches to evaluate dietary questionnaires: 1. Comparison of means vs. values derived from another source (e.g. NHANES) or vs. internal, based on another dietary assessment in same individuals

Approaches to evaluate dietary questionnaires: 2. Proportion of total intake accounted for by food items on FFQ if low the technique may not be comprehensive, but may still discriminate open ended section may be used to increase this

Approaches to evaluate dietary questionnaires: 3. Reproducibility made at two points in time Table 6-3 for nutrients; Table 6-4 for foods Range 0.5 to 0.7 vs. other variables (Table 6-5)

4. Validity typically assessed vs. a ‘Gold Standard’=? typical comparison is with – diet records (least correlated errors) – or multiple 24 hour recalls – or direct observation Table 6-6

5. Comparison with biochemical indicator measurement errors would be uncorrelated with dietary errors therefore qualitative validity is possible problem: other factors also control biochemical variable day-to-day fluctuation thus little utility as long term indicator lack of any indicator for many nutrients some nutrients are highly regulated (e.g. calcium) Table 6-13

6. Correlation with a physiologic response problem very few relationships are established example milk consumption in youth and bone density

7. Ability to predict disease example saturated fat and CHD green and yellow vegetables and lung cancer Problem: very few known relationships

Design of a Validation Study for Dietary Questionnaire 1. Population to study: ideally should be a random sample of study population 2. Choice of a comparison method: diet record since errors are independent (e.g. no memory required, open ended and direct measurement of portions can be done)

Design of a Validation Study for Dietary Questionnaire 3. Choice of an appropriate time frame: needs to be reflective of longer time period or to have replicate measures 4. The sequence of data collection in a validation study may be important. is it possible that the use of one measure could reflect on second measure? Could consider giving FFQ then record then FFQ

Design of a Validation Study for Dietary Questionnaire 5. Number of subjects and replicate measurements for a validation study: usual range for validity is r= 0.5 to 0.7 reasonable size is 100 to 200 people number of days per subject 2-4 diet records should be ok

Data analysis and presentation of validation studies: Can present - crude nutrient intakes adjusted for variables included in the study Notes: - dietary intakes are often skewed toward higher intakes so it may be necessary to log transform to increase Normality can use non-parametric correlation coefficients (e.g. Spearman Rank r) to compare categories of nutrient intake (Kappa= (Po-Pe)/ 1-Pe) use mean and standard deviation of difference between 2 evaluations

Recall of Remote Diet For many diseases it is hypothesized that the effect of diet may occur many years before the diagnosis. Validity can be assessed by resurvey of individuals who have completed diet questionnaires in the past. Several studies have done this Table 7-1

Recall of Remote Diet Problems: recalled diet may be heavily influenced by current diet correlations may be influenced by consistency in diet