Chapter 6 Reproducibility: duplicate measurements of the same individual in the same situation and...
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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
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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
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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
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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)
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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
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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
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6. Correlation with a physiologic response
• problem very few relationships are established
• example milk consumption in youth and bone density
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7. Ability to predict disease
• example saturated fat and CHD• green and yellow vegetables
and lung cancer• Problem: very few known
relationships
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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)
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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
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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
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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
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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
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Recall of Remote Diet
Problems:• recalled diet may be heavily
influenced by current diet• correlations may be influenced
by consistency in diet
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