Cognitive research enhances accuracy of food frequency questionnaire reports: results of an experimental validation study
Section snippets
Experimental Design
After stratified random assignment into 2 groups, study participants completed 30 days of criterion information, and then completed their respective FFQs (either the HHHQ or the test-DHQ) for the reference period of the previous month. Table 1 shows the items used to evaluate each question-design issue. Additional items were queried similarly on both FFQs (control items).
Instruments
The Daily Food Report (DFR), is a 1-page (2-sided) machine-scannable list of 88 food and drink items. Each participant filled in bubbles next to each item to indicate how many times the item was consumed that day. Items were placed on the DFR: a) if they were relevant to the 4 question-design issues under study; and b) if, based on other food intake data (USDA's Continuing Survey of Food Intakes of Individuals 1994–1996), it was expected that they would be consumed frequently enough within 30
Sample and Study Procedures
Reliably prompt mail was required to monitor daily completion of DFRs, so participants were sampled from a restricted geographical area. The sampling frame consisted of households with addresses listed in residential telephone directories in Washington, DC; Alexandria, Arlington, and Fairfax counties, Virginia; and Montgomery, Prince George's, and Frederick counties (excluding Frederick City), Md.
In August 1996 postcards were sent that provided brief information about the study and indicated
Analytical Procedures
On the DFRs, participants reported the number of times each item was consumed each day. On the FFQs, participants reported a rate of intake (ie, number of times per time period). Thus, the DFR information was continuous, whereas the FFQ information was categorical. To compare the two types of information required constructing new variables: category-converted DFR variables and continuous-converted FFQ variables.
Variables: DFR For each food, daily reported frequencies of intake on the DFR were
Statistical Analyses
With the categorical variables, three statistics were calculated to assess the accuracy of FFQ responses: the percentage of individuals whose FFQ responses agreed exactly with their category-converted DFR information, the percentage of those who never recorded consuming an item (on the DFR) and who reported never eating that item (on the FFQ), and the percentage of those who ever recorded consuming the food (on the DFR) and who reported consuming the food (on the FFQ). The continuity-adjusted χ2
Results
Of the 650 participants enrolled in the study, 623 (95.8%) returned at least 26 DFRs and the FFQ. Of these, 64% were female; 76% were white, 14% African-American, 4% Latino, and 6% other or unknown; 31% were ages 25 to 39 years, 43% ages 40 to 54, and 26% ages 55 to 70; 1% had less than high school education, 12% had no higher than high school education, and 86% had more than high school education; and 13% resided in Washington, DC, 31% in Virginia, 10% in rural Frederick, Md, and 44% in other
Comparability of HHHQ and Test-DHQ Samples
To examine comparability between participants completing the HHHQ and the test-DHQ, the demographic characteristics, FFQ completion rate, dietary intake recorded on the DFR instrument, and accuracy of FFQ reports for similarly worded foods were compared.
The HHHQ and test-DHQ groups were not significantly different in gender, age, race/ethnicity, educational level, or residence. Of the 623 participants in the study, 314 completed the HHHQ and 309 the test-DHQ, indicating that the greater length
Food Grouping: Single vs Separate Questions
Response accuracy of frequency reports for foods (Table 1) asked about in a single question vs asked about in separate questions were compared. In most but not all comparisons, the test-DHQ separated food question approach was superior to the HHHQ grouped food single question approach (Table 2).
Although there were no statistically significant differences between the two approaches in agreement for those respondents who did not consume the item, agreement for those who did consume the item was
Different Forms of Foods: Multiple Separate Questions vs Nesting
The HHHQ asks separate questions about frequency of consumption and portion size for different forms of certain foods. The test-DHQ uses a nesting approach in which questions about the frequency of consumption and portion size for the main food are asked, followed by questions about the relative proportions of time subordinate forms are consumed (Table 1).
In general, agreement statistics for the total samples indicate that frequency estimates for the main food were better with the nesting
Additions to Foods: Single Independent Question vs Nesting
The third design area examined—whether to ask about the use of an addition to food items in a single independent question (the HHHQ approach) or to nest questions about the use of the addition underneath each main food (the test-DHQ approach) (Table 1)—produced inconsistent results across the examined foods (Table 5). For milk on cereal, the test-DHQ nesting approach was somewhat better than the HHHQ single independent question approach. For salad dressing and mayonnaise, no differences between
Foods Conceptualized in Units: Frequency and Portion Size vs Frequency of Units
The last design issue examined was the utility of asking about foods in their usual serving units compared with asking about frequency and portion size (Table 1). Accuracy statistics for the two FFQs were generally similar (data not shown). Accuracy was relatively high for both instruments and for all items for nonconsumers and consumers. For example, for cups of coffee, correlations were 0.94 (HHHQ) and 0.92 (test-DHQ); percent agreement among nonconsumers was 95.3 (HHHQ) and 90.6 (test-DHQ);
Discussion
Many characteristics of FFQs likely affect reporting accuracy, including the number and nature of line items, wording, formatting, and response categories. The results of our study illustrate the complexity in the interplay of these characteristics.
The first question-design issue investigated in this study was the effect on accuracy of grouping several food items into a single question. In closed-ended dietary questionnaires like FFQs, it is impossible to ask about consumption of the entire
Applications/Conclusions
Cognitive research can reveal superior ways to ask about particular foods on an FFQ and other dietary assessment instruments. Dietitians should be aware of cognitive issues when selecting or modifying an FFQ for a particular target audience. The findings of this research were used to update and complete a version of the DHQ for public use (14).
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