The English version of the four-dimensional symptom questionnaire (4DSQ) measures the same as the original Dutch questionnaire: A validation study

Figures & data

Table 1. Cronbach's alpha and mean 4DSQ-scores of the Dutch and English speaking samples.

	Cronbach's alpha			Mean scores (SD)
4DSQ-scales	English	Dutch	P^a	English	Dutch	P^b
Distress	0.92	0.93	0.396	8.6 (7.2)	9.9 (8.5)	0.249
Depression	0.90	0.91	0.584	1.1 (2.3)	1.3 (2.7)	0.991
Anxiety	0.86	0.84	0.560	1.7 (3.4)	2.4 (3.8)	0.042
Somatization	0.82	0.85	0.280	6.2 (5.2)	8.2 (6.5)	0.001

^aZ-score test

^bMann–Whitney U test.

Table 2. Goodness-of-fit indices of the multi-group confirmatory factor analysis (one-factor model, configural invariance).

4DSQ-scales	CFI	TLI	RMSEA
Distress^a	0.998	0.997	0.039
Depression	1.000	1.000	0.000
Anxiety	1.000	1.002	0.000
Somatization^b	0.985	0.983	0.049

CFI, comparative fit index; TLI, Tucker–Lewis index; RMSEA, root mean square error of approximation.

^aResidual covariance allowed between pairs of items: 20–39, 47–48.

^bResidual covariance allowed between pairs of items: 2–5, 9–13, 12–13.

Table 3. Items identified with significant differential item functioning (DIF).

4DSQ-scales	Item no.	Short item description	Direction of DIF^a
Distress	22	Lack of energy	More likely
	47	Fleeting images of any upsetting event(s)	Less likely
Depression	28	Feel that everything is meaningless	Less likely
	35	Feel that there is no escape from your situation	More likely
Anxiety	27	Feel frightened	Less likely
	43	Afraid to travel on buses, streetcars, etc.	Less likely
Somatization	8	Headache	Less likely

^aEnglish-speaking patients more or less likely to endorse the item than Dutch patients.

Figure 1. Differential test functioning (DTF). Expected total scores as a function of the DIF-free scores and language. Standard (Dutch) cut-off points are indicated by dashed lines. In the anxiety graph (lower left panel) English total scores corresponding to the Dutch cut-off points are indicated as 11.94 and 7.32.

Table 4. Summary table of the effect of differential test functioning (DTF).

		95% CI				95% CI
4DSQ-scales	DTFR^a	Lower Bound	Upper Bound	P	dDTF^b	Lower Bound	Upper Bound
Distress	0.08	–0.07	0.23	0.300	0.01	–0.01	0.03
Depression	0.05	–0.07	0.17	0.407	0.02	–0.03	0.07
Anxiety	–0.22	–0.32	–0.13	0.000	–0.07	–0.09	–0.04
Somatization	–0.22	–0.35	–0.09	0.001	–0.04	–0.07	–0.02

95% CI, 95% confidence interval.

^aDTFR represents the effect of DTF on the mean total score of the English group.

^bd_DTF represents the DTF effect size (i.e., DTFR/standard deviation of the English group).