Test-retest reproducibility of a food frequency questionnaire (FFQ) and estimated effects on disease risk in the Norwegian Women and Cancer Study (NOWAC)

Table 2 Measures of reproducibility for single questions about consumption frequency in the test and retest food frequency questionnaire (FFQ)

		Agreement, %		Misclassification, %			Symmetry of misclassification, %			Kappa
Food item (response options)	n	Total	Never/rarely^a	± 1 Categ.^b	± 2 Categ.^b	Extr.^c	Retest<Test	Retest>Test	Difference	Simple	Weighted
Potatoes (7)	1452	62	1	25	9	0.0	21	17	4	0.48	0.61
Carrots (7)	1406	43	1	42	11	0.0	35	22	13	0.30	0.54
Salad (7)	1169	41	3	41	12	0.0	22	37	-15	0.28	0.52
Swede (7)	1134	49	7	38	8	0.0	31	20	11	0.32	0.52
"Other vegetables" (7)	865	34	8	35	18	0.5	34	32	2	0.20	0.37
Oranges (7)	1107	39	5	34	20	0.1	50	12	38	0.25	0.47
"Other fruits" (7)	978	39	4	34	17	0.2	29	33	-4	0.25	0.42
Whole milk (6)	710	85	78	11	3	0.1	8	6	2	0.52	0.61
Meat, reindeer (6)	1339	86	75	11	1	0.6	8	6	2	0.58	0.57
Roast (beef, pork, lamb) (5)	1280	54	13	37	7	0.1	26	20	6	0.33	0.42
Meat, chops (5)	1301	62	10	35	4	0.0	15	24	-9	0.42	0.52
"Other meat dishes" (5)	921	40	11	41	15	0.2	31	28	3	0.21	0.32
Cod/coalfish/haddock/pollack (6)	1362	48	2	41	9	0.0	29	22	7	0.32	0.51
Salmon/trout (6)	1225	53	5	39	7	0.0	22	25	-3	0.35	0.48
Fish liver (5)	1229	78	43	18	3	0.2	12	10	2	0.68	0.77
Shellfish (4)	1374	70	35	27	2	0.1	13	17	-4	0.54	0.62
Beer (7)	1005	63	41	29	6	0.0	14	23	-9	0.46	0.61
Wine (7)	1137	46	13	40	13	0.0	19	35	-16	0.34	0.60
Chocolate (6)	1402	57	18	35	6	0.0	26	17	9	0.42	0.57
Cod liver oil capsules, winter (5)	156	76	3	13	3	5.1	9	15	-6	0.33	0.45
Cod liver oil capsules, rest of year (5)	130	69	15	12	4	11.5	15	16	-1	0.44	0.53

^aLowest response category in the FFQ, ^bCategories: ± 1 or ± 2 from the line of agreement in the contingency table
^cExtremes: test-retest responses in the lowest and highest categories

ISSN: 1475-2891