Application and interpretation of multiple statistical tests to evaluate validity of dietary intake assessment methods

Table 4 Statistical test outcomes and interpretation for energy and nutrient intakes derived from the test data set ¹

Nutrient	Spearman correlation (r value)	Wilcoxon signed rank test (P value)	Percentage difference (%)	Cross-classification (Tertiles)		Weighted Kappa statistics (value)	Bland – Altman ^2,3 Spearman Correlation (r value)
	Association (strength & direction)	Agreement	Agreement (size & direction of error)	Agreement (including chance)		Agreement (excluding chance)	Presence, direction and extent of bias
		Agreement	Agreement (size & direction of error)	% in same tertile	% in opposite tertile	Agreement (excluding chance)	Presence, direction and extent of bias
Level of validation	Individual	Group	Group	Individual	Individual	Individual	Group
Energy (kJ)	0.26	P > 0.05	−9.8	46.8	19.2	0.20	P < 0.001
Validly interpretation	Acceptable	Good	Good	Poor	Poor	Acceptable	Biased
Protein (g)	0.23	P < 0.01	−19.1	42.6	23.4	0.12	P < 0.05
Validly interpretation	Acceptable	Poor	Acceptable	Poor	Poor	Poor	Biased
Fat (g)	0.01	P > 0.05	−6.9	34.0	31.9	−0.01	P > 0.05
Validly interpretation	Poor	Good	Good	Poor	Poor	Poor	Not biased
Carbohydrates (g)	0.40	P > 0.05	−1.4	50.0	17.4	0.25	P < 0.01
Validly interpretation	Acceptable	Good	Good	Good	Poor	Acceptable	Biased
Folate (mcg)	0.40	P < 0.05	33.0	53.2	8.5	0.30	P < 0.01
Validly interpretation	Acceptable	Poor	Poor	Good	Good	Acceptable	Biased
Vitamin A (mcg)	0.15	P < 0.01	−22.9	34.0	14.9	0.03	P > 0.05
Validly interpretation	Poor	Poor	Poor	Poor	Poor	Poor	Not biased
Iron (mg)	0.38	P > 0.05	−24.8	51.1	23.4	0.29	P < 0.01
Validly interpretation	Acceptable	Good	Poor	Good	Poor	Acceptable	Biased

¹Energy and nutrient intake derived from a QFFQ and four repeated 24-hour recalls conducted in 18–65 year old adults (n = 47, 11 males & 36 females) in a rural area in the Eastern Cape.
²% in LOA: Energy: 93.6%; protein: 95.7%, fat: 97.9%, carbohydrate: 95.7%; folate: 95.7%, vitamin A: 89.4%; iron: 98%.
³Upper & lower limits of agreement (LOA) and DRI for females aged 18 to 55: energy (kJ): −13406 & 10694 (EER: 10093 kJ); protein (g): −94.6 & 59.4 (RDA: 46 g); fat: −125.4 & 96.6 (no DRI); folate (mcg): −649 & 925.8 (RDA: 400mcg); vitamin A (mcg): −876 &576.4 (RDA: 700mcg); iron (mg): −22,4 &10.4 (RDA: ????mg).
Interpretation criteria for statistical tests.
Wilcoxon signed rank test: Good: p > 0.05; Poor: ≤ 0.05 [9].
Percentage difference: Good: 0.0 – 10.9%; Acceptable: 11.0 – 20.0%; Poor: > 20.0% [92].
Correlations coefficient (Spearman): Good: ≥ 0.50; Acceptable: 0.20 – 0.49; Poor < 0.20[2]
Cross-classification (Tertiles) (% in same tertile): Good: ≥ 50%; Poor: < 50% [2].
Cross-classification (% in opposite tertile): Good: ≤ 10%, Poor: > 10% [2].
Weighted Kappa statistics: Good: ≥ 0.61; Acceptable: 0.20 – 0.59; Poor: < 0.20 [2].
Bland-Altman - Correlation coefficient (Spearman): Good: P > 0.05; Poor: P ≤ 0.05 [2].

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