Rasch analysis of the Western Ontario Osteoarthritis of the Shoulder index – the Danish version

Figures & data

Table 1 Western Ontario Osteoarthritis of the Shoulder index

Section A: physical symptoms

1. How much pain do you experience in your shoulder with movement?

2. How much constant, nagging pain do you have in your shoulder?

3. How much weakness do you experience in your shoulder?

4. How much stiffness do you experience in your shoulder?

5. How much grinding do you experience in your shoulder?

6. How much is your shoulder affected by the weather?

Section B: sports/recreation/work

7. How much difficulty do you experience working or reaching above shoulder level?

8. How much difficulty do you experience with lifting objects (eg, grocery bags, and garbage cans) below shoulder level?

9. How much difficulty do you experience doing repetitive motions below shoulder level, such as raking, sweeping, and washing floors because of your shoulder?

10. How much difficulty do you experience pushing or pulling forcefully because of your shoulder?

11. How troubled are you by an increase in pain in your shoulder after activities?

Section C: lifestyle

12. How much difficulty do you have sleeping because of your shoulder?

13. How much difficulty have you experienced with styling your hair because of your shoulder?

14. How much difficulty do you have maintaining your desired level of fitness because of your shoulder?

15. How much difficulty do you experience reaching behind to tuck in a shirt, get a wallet from your back pocket or do up clothing because of your shoulder?

16. How much difficulty do you have dressing or undressing?

Section D: emotions

17. How much frustration or discouragement do you feel because of your shoulder?

18. How worried are you about what will happen to your shoulder in the future?

19. How much of a burden do you feel you are on others?

Figure 1 Scree plot.

Notes: The scree plot shows the results of the PCA for the pooled osteoarthritis and fracture data. It illustrates the distribution of eigenvalues among the 19 contrasts.
Abbreviation: PCA, principal component analysis.

Figure 2 Histograms of item scores.

Notes: The histograms of each item on WOOS depict the distribution of the item scores in the common study population. The counts indicate the number of responses.
Abbreviation: WOOS, Western Ontario Osteoarthritis of the Shoulder.

Table 2 Summary of the category structure of the Danish version of WOOS

Category label/score	Observed count	Observed count (%)	Observed average	Outfit mean square	Threshold
0	11,481	71	−1.57	0.94	None
1	4,612	29	0.88	1.08	0

Abbreviation: WOOS, Western Ontario Osteoarthritis of the Shoulder.

Figure 3 Wright map of the WOOS scale for osteoarthritis patients.

Notes: People (n=847) are shown on the left side, and the 19 items are shown on the right side. Each “#” represents 23 patients, and each “.” represents 1–22 patients. The logit scale represents quality of life. Positive scores indicate bad outcomes, and negative scores indicate good outcomes. The mean item difficulty was defined as zero on the scale.
Abbreviation: WOOS, Western Ontario Osteoarthritis of the Shoulder; M, mean; S, one standard deviation; T, two standard deviations.

Table 3 Item statistics for the WOOS for osteoarthritis patients

Item	Measure	Infit MNSQ	Outfit MNSQ
2	1.40	0.97	0.81
5	1.40	1.12	1.60
19	1.05	0.93	0.97
6	0.89	1.44	2.46
1	0.51	1.00	0.96
8	0.50	1.07	1.10
4	0.45	0.99	0.80
12	0.33	1.00	0.99
17	0.28	0.86	0.69
18	0.07	0.98	0.91
14	−0.04	0.89	0.82
16	−0.15	0.96	0.83
13	−0.37	1.04	1.01
9	−0.42	0.96	0.90
11	−0.65	0.88	0.74
10	−0.82	0.95	0.89
3	−0.86	0.91	0.88
15	−1.21	1.07	1.10
7	−2.36	1.06	0.90

Abbreviations: MNSQ, mean square; WOOS, Western Ontario Osteoarthritis of the Shoulder.

Table 4 Item statistics for the WOOS for fracture patients

Item	Measure	Infit MNSQ	Outfit MNSQ
6	2.11	1.30	1.81
2	1.81	0.92	1.00
5	1.75	1.00	1.05
12	1.30	1.11	1.32
1	0.85	0.91	0.79
19	0.85	1.01	0.86
8	0.61	1.06	1.04
17	0.45	0.86	0.92
18	0.34	0.91	1.01
4	−0.02	1.08	1.00
11	−0.17	0.86	0.70
14	−0.19	0.93	0.93
9	−0.39	0.95	0.87
16	−0.44	0.99	0.90
10	−0.84	0.92	0.79
15	−1.12	1.07	1.09
3	−1.20	1.01	1.05
13	−1.73	1.11	1.13
7	−3.95	1.11	1.28

Abbreviations: MNSQ, mean square; WOOS, Western Ontario Osteoarthritis of the Shoulder.

Figure 4 Differential item functioning plot.

Note: These graphs show the item difficulty measures for FR and OA.
Abbreviations: DIF, differential item functioning; FR, fracture; OA, osteoarthritis.

Table 5 Significant DIF between fracture patients and osteoarthtitis patients

Item	DIF size
Item 4
FR patients experience more stiffness	−0.51
Item 6
FR patients are less affected by the weather	1.12
Item 7
FR patients have more difficulty working or reaching above shoulder level	−1.32
Item 11
FR patients are less troubled by an increase in pain after activities	0.50
Item 12
FR patients have less difficulty sleeping	0.90
Item 13
FR patients experience more difficulty styling their hair	−1.30

Notes: This table shows the item difficulties for FR that differed from the baseline OA item difficulties by a DIF size >0.50 or <−0.50 logits. These values indicate how much FR patients deviate on an item score when compared with OA. The positive DIF sizes indicate high item difficulty, which means that the FR patients had better experiences than the OA sample regarding the specific issues examined in the item. Negative DIF sizes indicate the opposite. All of the DIF sizes listed in the table are considered statistically significant because they have t-scores >2.

Abbreviations: DIF, differential item functioning; FR, fracture; OA, osteoarthritis.