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
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