Table shows that the selected characteristics of the respondents (n = 1488) and the total sample invited for the reproducibility study (n = 1994) were similar.
Mean (range or SDa) or proportion for selected characteristics of the respondents and the total sample invited to the reproducibility study
Reproducibility of single food items
The food items in Table were selected to illustrate the range of values for all the frequency questions in the FFQ. Reindeer meat and whole milk had the highest proportions of total agreement (≥85%), but also the highest agreement for the "never/rarely" consumption category (75–78%). Cod liver oil capsules, in winter and in the rest of the year, had the highest proportions of extreme misclassification, 5% and 12%, respectively. Oranges and the open categories "other vegetables", "other meat dishes" and "other fruit" had the lowest proportions of total agreement (34–40%), high proportions in the ± 2 adjacent categories (15–20%), and the lowest values for simple Kappa (0.20–0.25). Weighted Kappa was always higher, except for in reindeer meat where the weighed value was a little lower (0.57 compared to 0.58). The difference between the misclassification above and below the table diagonal was negative if more subjects reported a higher frequency in the retests compared to the test. The largest differences were observed for wine (-16%) and salad (-15%), followed by beer and meat chops (-9%). The difference was positive if more subjects reported a lower frequency in the retest compared to the test. The largest values were observed for oranges (38%), followed by carrots, swede, and chocolate (9–13%). Within the same food group (e.g. meat) the reports of some items increased (meat chops), while others decreased (roast). Similar for fish, salmon and shellfish increased while cod/coalfish decreased.
Measures of reproducibility for single questions about consumption frequency in the test and retest food frequency questionnaire (FFQ)
When the frequency questions were divided into categories, frequencies with no additional portion size question had a slightly higher median value for total agreement (65% versus 56%) and weighted Kappa (0.57 versus 0.51) than frequencies with a related portion size question. The median values for the portion size questions were 61% for total agreement and 0.47 for weighted Kappa (results not shown).
Reproducibility of food groups
The mean daily intake of most food groups was slightly lower in the retest (Table ). As seen from the 95% CI for the mean differences, significant decreases were observed for "potatoes", "fruits", "bread, crisp bread, breakfast cereal", "whole fish and shellfish", and "fat on bread". However, significant increases were observed for "red meat and chicken", "eggs", "orange juice, soft drinks, diluted syrups" and "alcoholic beverages". The limits of agreement were generally wide. Pearson's r ranged from 0.50 ("condiments and sauces for fish") to 0.79 ("cod liver oil supplements") with a median value of 0.66. ICC(1, 1) and ICC(3, 1) were very similar to r. The largest difference was seen for "alcoholic beverages" with r = 0.68, ICC(1, 1) = 0.66, and ICC(3, 1) = 0.67 (results not shown). Spearman's rs ranged from 0.55 ("bread, crisp bread and breakfast cereal") to 0.80 ("cream desserts and milk based puddings") with a median value of 0.70. There were some differences between rs and the other reliability coefficients, and for "dairy products" and "alcoholic beverages" rs was somewhat higher.
Measures of reproducibility for the intake of food groups (g/day) in the test and retest food frequency questionnaire (FFQ), n = 1370
Reproducibility of energy and nutrients
The mean daily intake was significantly lower in the retest for energy, protein, fat (total and polyunsaturated), and total carbohydrate, but not the corresponding energy percentages (Table ). Significant lower intakes were also observed for dietary fibre, retinol, vitamin C, and calcium. The intake was significantly higher in the retest for alcohol, and percent energy from both alcohol and sugar. Pearson's r ranged from 0.55 (calcium) to 0.78 (vitamin E), with a median value of 0.67. ICC (1, 1), and ICC (3, 1) were again very similar to r (results not shown). The range for Spearman's rs was 0.60 (protein, calcium) to 0.78 (% energy from alcohol) with a median of 0.67. rs was slightly higher than the other reliability coefficients for calcium, alcohol, and % energy from alcohol, which is consistent with the higher values observed for the food groups "dairy products" and "alcoholic beverages".
Measures of reproducibility for the daily intake of energy and selected nutrients in the test and retest food frequency questionnaire (FFQ), n = 1370
Impact on diet-disease associations
In our example to demonstrate the regression calibration method, high blood pressure was negatively associated with alcohol intake (Table ). The uncorrected estimates based on the test, the retest, and the test-retest mean were biased towards the null value (referred to as attenuation) having values closer to OR = 1 than the estimate corrected by regression calibration. The effect is more clearly seen for an increase of 10 g of alcohol per day (a little less than the amount in a standard glass of wine in NOWAC) with OR = 0.53 for the test, OR = 0.49 for the retest, OR = 0.45 for the test-retest mean, and OR = 0.38 for the calibrated intake.
Table 5 Odds ratio (OR) estimates and confidence intervals (CIs) for high blood pressure (yes = 301/no = 712) in relation to alcohol intake (g/day change) in the food requency uestionnaire (FFQ). Estimates are compared for the test, the retest, the test-retest (more ...)