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To investigate item non-response in a postal food-frequency questionnaire (FFQ), and to assess the effect of substituting/imputing missing values on dietary intake levels in the Norwegian Women and Cancer study (NOWAC). We have adapted and probably for the first time applied k nearest neighbours (KNN) imputation to FFQ data.
Data from a recent reproducibility study were used. The FFQ was mailed twice (test–retest) about 3 months apart to the same subjects. Missing responses in the test FFQ were imputed using the null value (frequencies = null, amount = smallest), the sample mode, the sample median, KNN, and retest values.
A methodological substudy of NOWAC, a national population-based cohort.
A random sample of 2000 women aged 46–75 years was drawn from the cohort in 2002 (response 75%). The imputation methods were compared for 1430 women who completed at least 50% of the test FFQ.
We imputed 16% missing values in the overall test data matrix. Compared to null value imputation, the largest differences in estimated dietary intake were seen for KNN, and for food items with a high proportion of missing. Imputation with retest values increased total energy intake, indicating that not all missing values are caused by respondents failing to specify no consumption, and that null value imputation may lead to underestimation and misclassification.
Missing values in FFQs present a methodological challenge. We encourage the application and evaluation of newer imputation methods, including KNN, which may reduce imputation errors and give more accurate intake estimates.
To assess the relative validity of the second version of a quantitative food-frequency questionnaire (QFFQ), designed to measure the habitual food and nutrient intake in one season in rural populations in Western Mali, West Africa.
The dietary intake during the previous week was assessed with the 164-item QFFQ administered by interview. This was compared with the intake from a 2-day weighed record (WR) with weighed recipes.
The village of Ouassala in the Kayes region, Western Mali.
Thirty-four women and 36 men aged 15–45 years, from 29 households.
The QFFQ gave a lower intake of lunch and dinner and a higher intake of snacks than the WR. The discrepancies were larger for women than for men. The median proportion of subjects classified in the same quartile of intake was 29% for food groups and 36% for energy and nutrients. For classification into extreme opposite quartiles, the median proportion was 6% for food groups and 7% for energy and nutrients. Spearman's rank correlation for energy and nutrients ranged from 0.16 (% energy from protein) to 0.62 (retinol equivalents).
The second version of the QFFQ tends to underestimate total food weight. The methods used for estimating food portion size should therefore be applied with caution. The changes made from the first version had little effect. The ability to rank subjects according to dietary intake is similar with both versions. The improved layout of the new QFFQ makes it a more user-friendly tool for comparing dietary intake between population groups and for measuring changes over time.
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