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Chapter 6

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

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Chapter 6

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  1. 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

  2. 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

  3. 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

  4. 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)

  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

  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

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

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

  9. 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)

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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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