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

Questionnaire1. 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

Questionnaire3. 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

Questionnaire5. 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 intakesadjusted 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