Diagnosing depression in primary care: a Rasch analysis of the Major Depression Inventory

Figures & data

Table 1. Model fit statistics for MDI items.

Action	Analysis	Overall model fit	Items fit residual Mean (SD)	Persons fit residual Mean (SD)	PSI	Significant t-tests (%) (CI 95%)
Case-finding, N = 350	1	χ^2(90) = 117.32, p = 0.028	0.30 (2.06)	−0.30 (1.23)	0.88	7.65 (5.15–11.03)
Rescoring all items, N = 350	2	χ^2(90) = 126.40, p = 0.007	0.32 (2.08)	−0.34 (1.33)	0.88	7.65 (5.15–11.03)
Excluding item 9, N = 350	3	χ^2(90) = 83.94, p = 0.389	0.26 (1.50)	−0.33 (1.19)	0.88	4.53 (2.64–7.32)
Core symptoms: items 1-3, N = 338	4	χ^2(24) = 29.92, p = 0.187	−0.29 (0.76)	−0.43 (0.96)	0.76	6.23 (3.88–9.34)
Diagnostic dichotomization, N = 276	5	χ^2(80) = 87.25, p = 0.271	−0.03 (1.43)	−0.06 (0.69)	0.72	2.83 (1.26–5.63)

χ²(df): chi-square (degrees of freedom); p: probability; SD: standard deviation; PSI: person separation index (with extremes); CI: Confidence Interval.

All analyses are separate analyses. Subsequent reduction in N is due to exclusion of extreme scores from the analysis, as they provide no information on rank ordering of persons or items.

Table 2. Individual item fit for MDI items.

	Response category^a n(%)						Logit location	s.e.	Fit residual	χ^2	d.f.	χ^2 probability
MDI item	0	1	2	3	4	5
1 Have you felt low in spirits or sad?	12 (3)	75 (21)	34 (10)	71 (20)	120 (34)	38 (11)	−0.347	0.052	−1.550	11.537	9	0.241
2 Have you lost interest in your daily activities?	37 (11)	61 (17)	39 (11)	54 (15)	117 (33)	42 (12)	−0.080	0.048	−1.313	12.293	9	0.197
3 Have you felt lacking in energy and strength?	13 (4)	41 (12)	33 (9)	43 (12)	127 (36)	93 (27)	−0.721	0.052	−0.295	6.855	9	0.652
4 Have you felt less self-confident?	46 (13)	60 (17)	42 (12)	58 (17)	82 (23)	62 (18)	−0.064	0.045	0.376	8.072	9	0.527
5 Have you had a bad conscience or feelings of guilt?	59 (17)	65 (19)	36 (10)	57 (16)	80 (23)	53 (15)	0.094	0.044	0.069	6.019	9	0.738
6 Have you felt that life wasn’t worth living?	140 (40)	85 (24)	34 (10)	41 (12)	30 (9)	20 (6)	0.842	0.046	−0.295	5.244	9	0.813
7 Have you had difficulty in concentrating, e.g. when reading the newspaper or watching television?	65 (19)	81 (23)	42 (12)	65 (19)	68 (19)	29 (8)	0.346	0.046	0.665	7.720	9	0.563
8 Have you felt very restless?/ Have you felt subdued or slowed down?	20 (6)	61 (17)	36 (10)	68 (19)	110 (31)	55 (16)	−0.339	0.051	−2.064	16.659	9	0.054
9 Have you had trouble sleeping at night?	48 (14)	74 (21)	39 (11)	41 (12)	74 (21)	74 (21)	−0.053	0.043	4.810	35.106	9	0.000
10 Have you suffered from reduced appetite?/ Have you suffered from increased appetite?	81 (23)	78 (22)	41 (12)	48 (14)	61 (17)	61 (12)	0.323	0.043	2.576	7.815	9	0.553

^a Response categories: (0) at no time; (1) some of the time; (2) slightly less than half of the time; (3) slightly more than half the time; (4) most of the time; (5) all the time.

Figure 1. The MDI category probability curves for item 1 ‘feeling sad’ displaying disordered six-point response categories and corrected five-point response categories.

Figure 2. All 10 MDI items displayed disordered six-point response categories and corrected five-point response categories (A). Dichotomization according to diagnostic algorithm revealed ordered response categories (B). All items are sorted in location order.

Figure 3. The person-item threshold map for the MDI items for the primary care sample. A total of 57% of persons were classified as clinically depressed according to the MDI algorithm.