Table 2. Psychometric properties of the items in the knowledge about dietary supplements (KaDS) questionnaire.
| No | English translation of each retained statement | True (T) or False (F) | Mean grade by competent judges1 (SD) |
Familiarity2 | Difficulty3 | Discrimination4 | Exploratory Factor Analysis5 | Confirmatory Factor Analysis6 Parameter estimate (95%CI) | Reference supporting the statement | |
|---|---|---|---|---|---|---|---|---|---|---|
| Factor 1 loadings | Factor 2 loadings | |||||||||
| 7-item “general” subscale (Kuder-Richardson 20 = 0.77; skewness = -0.17, 95%CI: -0.50 to 0.15; kurtosis = -1.22, 95%CI: -1.87 to -0.58) | ||||||||||
| 1 | Before being marketed, dietary supplements must be tested for efficacy and safety. | F | 4.67 (0.71) | 49/57 (86%) | 100/220 (45.5%) | 0.67 | 0.81 | 0.02 | 0.40 (0.34 to 0.46) p<0.0001 | [16, 35–37] |
| 2 | An ingredient may be sold both as a medicine or as a dietary supplement. | T | 4.56 (0.73) | 41/57 (72%) | 81/220 (36.8%) | 0.20 | 0.25 | -0.37 | 0.08 (0.01 to 0.15) p = 0.012 | |
| 3 | The quality of dietary supplements is routinely tested before being marketed. | F | 4.44 (1.01) | 42/56 (75%) | 93/219 (42.5%) | 0.59 | 0.76 | 0.11 | 0.36 (0.30 to 0.42) p<0.0001 | |
| 4 | The packaging of dietary supplements must contain information on possible adverse effects resulting from their use. | F | 4.22 (1.39) | 48/57 (84%) | 112/218 (51.4%) | 0.52 | 0.68 | -0.24 | 0.28 (0.21 to 0.35) p<0.0001 | |
| 5 | Dietary supplements are food. | T | 4.78 (0.44) | 41/56 (73%) | 115/219 (52.5%) | 0.28 | 0.39 | -0.14 | 0.14 (0.07 to 0.21) p = 0.0002 | |
| 6 | Dietary supplement registration requires assessing the composition of the product by the appropriate supervisory body. | F | 4.78 (0.44) | 46/57 (81%) | 114/220 (51.8%) | 0.63 | 0.76 | -0.05 | 0.35 (0.29 to 0.41) p<0.0001 | |
| 7 | All dietary supplements sold in pharmacies have been tested for safety. | F | 4.67 (0.71) | 43/56 (77%) | 73/219 (33.3%) | 0.58 | 0.74 | 0.13 | 0.34 (0.28 to 0.40) p<0.0001 | |
| 10-item “specific” subscale (Kuder-Richardson 20 = 0.54; skewness = 0.23, 95%CI: -0.09 to 0.56; kurtosis = 0.04, 95%CI: -0.61 to 0.69) | ||||||||||
| 8 | Taking vitamin and mineral supplements prevents diseases in healthy people. | F | 4.44 (1.13) | 49/57 (86%) | 94/220 (42.7%) | 0.25 | 0.24 | 0.27 | 0.19 (0.10 to 0.27) p<0.0001 | [2, 6–10, 35, 36] |
| 9 | In the elderly, taking vitamin D reduces the risk of bone fractures. | F | 4.44 (1.01) | 45/56 (80%) | 154/219 (70.3%) | 0.32 | -0.13 | 0.65 | 0.12 (0.05 to 0.20) p = 0.0010 | [38, 39] |
| 10 | In the elderly, the use of magnesium preparations prevents muscle cramps. | F | 4.29 (0.76) | 45/57 (79%) | 169/219 (77.2%) | 0.21 | -0.04 | 0.60 | 0.17 (0.09 to 0.25) p<0.0001 | [40, 41] |
| 11 | Taking dietary supplements containing calcium reduces the risk of bone fractures in the elderly. | F | 4.50 (0.53) | 45/57 (79%) | 150/220 (68.2%) | 0.28 | 0.16 | 0.55 | 0.22 (0.14 to 0.33) p<0.0001 | [42, 43] |
| 12 | The use of multivitamin preparations protects against heart diseases. | F | 4.50 (0.84) | 35/57 (61%) | 41/218 (18.8%) | 0.25 | 0.22 | 0.39 | 0.14 (0.07 to 0.21) p<0.0001 | [44, 45] |
| 13 | The use of antioxidants prevents the development of cancer. | F | 4.00 (1.32) | 35/57 (61%) | 134/220 (60.9%) | 0.26 | 0.02 | 0.42 | 0.14 (0.05 to 0.22) p = 0.0020 | [10, 11] |
| 14 | Regular use of vitamin C reduces the risk of catching a cold. | F | 4.50 (0.93) | 48/57 (84%) | 176/219 (80.4%) | 0.32 | 0.35 | 0.47 | 0.20 (0.13 to 0.27) p<0.0001 | [8, 46] |
| 15 | Taking excessive amounts of magnesium supplements can cause diarrhea and nausea.7 | T | 4.12 (0.99) | 34/57 (60%) | 44/220 (20.0%) | 0.12 | 0.01 | 0.03 | 0.04 (-0.03 to 0.11) p = 0.24 | [47] |
| 16 | Vitamin C naturally present in food is better assimilated than synthetic.7 | F | 4.44 (0.88) | 52/57 (91%) | 196/220 (89.1%) | 0.09 | -0.04 | 0.10 | 0.03 (-0.03 to 0.08) p = 0.35 | [48] |
| 17 | People with kidney disease should not use high doses of vitamin C.7 | T | 4.22 (0.83) | 29/57 (51%) | 80/220 (36.4%) | 0.18 | 0.05 | 0.23 | 0.09 (0.00 to 0.17) p = 0.044 | [49, 50] |
| Eigenvalue | 3.31 | 2.00 | ||||||||
| Variance explained | 19.5% | 11.8% | ||||||||
1 A grade by competent judges was ranging from “5” (very relevant and important for the public to know) to “1” (very irrelevant and unimportant).
2 Familiarity–defined as 1-fraction of “don’t know” answers.
3 Difficulty–defined as fraction of wrong answers.
4 Discrimination–corrected item-total correlation.
5 Varimax rotation was used to enhance the dissimilarity of the extracted factors
6 Confirmatory factor analysis: χ2(118) = 225.3, p < 0.0001, χ2/df = 1.91, adjusted goodness of fit index = 0.85, Tucker-Lewis index = 0.78, Bollen’s Incremental Fit Index = 0.81, Bentler’s Comparative Fit Index = 0.81, Root Mean Square Error of Approximation = 0.070 (90%CI: 0.058–0.083), Standardized Root Mean Square Residual = 0.078. The subscales: “general” and “specific” were correlated with each other: CFA parameter estimate for the correlation = 0.29 (95%CI: 0.11–0.48), p = 0.0019.
7 These items were retained in the final questionnaire despite unfavourable psychometric properties in order to restore content validity.
Internal consistency of the questionnaire estimated by the McDonald’s omega = 0.79.