Background
Methods
Design
Participants and data collection
Measures
SF36
Validation measures
Grouping factors
Statistical analysis
Testing assumptions: unidimensionality
DIF testing
Ordinal logistic regression model
Significance testing and item level effect sizes
Purification
Effects of covariates
Examination of the impact of DIF at the measure and subscale level
The validity and reliability of DIF-free measure
Power
Results
Demographics
Gender (male, %, (n)) | 43.3% (330) |
Age (Mean years, s.d.) | 69.5 (10.0) |
Marital status (married, %, (n)) | 72% (543) |
Ethnicity (white, &, (n)) | 99.1% (750) |
Paid employment (yes, %, (n)) | 33.7% (247) |
Social class (%, (n)) | |
I | 5.7% (43) |
II | 33.8% (256) |
IIINM | 19.0% (144) |
III M | 23.4% (177) |
IV | 14.7% (111) |
V | 3.4% (26) |
Townsend quintiles (lower = most affluent) | |
20% | -2.7 |
40% | -1.9 |
60% | -0.80 |
80% | 1.4 |
Mood (mod or severe/none; %, (n)) | 30%/70% (218/534) |
BMI (mean(s.d.)) | 29.2 (5.3) |
Affected joints (n) | |
Hip OA | 487 |
Knee OA | 612 |
Both hip and Knee OA | 336 |
Hip OA only | 151 |
Knee OA only | 276 |
No of affected hip/knee joints (1/2v3/4;%, (n)) | 57%/43% (218/435) |
SF-36 subscales^ | |
Physical functioning (mean(s.d.) | 50.0% (29.0) |
Pain (mean(s.d.) | 42.3% (20.0) |
Role physical (mean(s.d.) | 39.2% (41.7) |
Social (mean(s.d.) | 71.8% (28.3) |
Role emotional (mean(s.d.) | 57.7% (42.9) |
Mental health (mean(s.d.) | 73.9% (18.1) |
Vitality (mean(s.d.) | 50.8% (21.3) |
General health (mean(s.d.) | 59.6% (21.2) |
Testing assumptions: unidimensionality
SF-36 subscale | Factor1 eigenvalue | % variance | Factor2 eigenvalue | % variance | Factor3 eigenvalue | % variance | MAP: number of dimensions | N |
---|---|---|---|---|---|---|---|---|
Physical | 7.83 | 78.3 | 0.50 | 5.0 | 0.47 | 4.7 | 1 | 734 |
Role phys | 3.65 | 91.3 | 0.15 | 3.8 | 0.13 | 3.3 | 1 | 749 |
Role emot | 2.71 | 90.3 | 0.19 | 6.3 | 0.10 | 3.3 | 1 | 742 |
Mental | 3.36 | 67.1 | 0.67 | 13.3 | 0.46 | 9.2 | 1 | 753 |
Vitality | 2.94 | 73.4 | 0.61 | 15.2 | 24.1 | 6.0 | 1 | 754 |
Gen health | 3.27 | 65.5 | 0.65 | 13.0 | 0.56 | 11.3 | 1 | 756 |
DIF items
Item | Age | Gender | Social class | Social deprivation | Employ | Mood | BMI | No. joints | Hip/Knee |
---|---|---|---|---|---|---|---|---|---|
Physical functioning
| |||||||||
PF1 vigorous activities |
19.21
NU
| 2.26 | 1.26 | 5.24 | 3.99 | 0.23 |
13.80
NU
| 0.99 | 0.93 |
PF2 moderate activities | 1.98 | 1.32 | 5.90 | 5.15 | 1.47 | 3.93 | 0.21 | 4.77 | 1.50 |
PF3 lift/carry groceries | 0.53 |
29.08
U
*
| 1.13 | 0.39 | 2.40 | 1.90 | 1.03 | 2.53 | 2.98 |
PF4 several flight stairs | 1.96 | 10.17 | 4.24 | 5.17 | 5.14 | 4.94 | 2.13 | 3.00 | 5.63 |
PF5 one flight stairs | 2.87 | 0.49 | 2.33 | 1.91 | 0.79 | 0.54 | 3.31 | 1.44 | 1.67 |
PF6 bend/knee/stoop |
16.73
NU
| 5.58 | 4.74 | 2.31 | 4.42 | 1.21 | 5.43 | 1.024 | 0.21 |
PF7 walk more mile | 4.04 | 2.63 | 2.85 | 0.92 | 2.75 | 2.09 | 0.85 | 1.39 | 0.65 |
PF8 walk half mile | 0.37 | 0.41 | 3.37 | 3.01 | 1.02 | 1.48 | 0.50 | 0.57 | 7.25 |
PF9 walk 100 yards | 10.73 | 0.20 | 2.87 | 6.53 | 0.00 | 3.01 | 2.40 | 4.07 | 0.81 |
PF10 bathing or dressing | 1.01 | 4.36 | 2.04 | 3.71 | 11.01 |
12.67
| 6.26 | 1.80 | 1.49 |
Pain
| |||||||||
P1 intensity of bodily pain | 8.55 | 0.24 | 8.21 | 1.60 |
32.45
NU
*
| 4.65 | 4.10 | 0.73 | 4.59 |
P2 extent pain interfere normal work | 5.78 | 7.66 |
10.34
U/NU
*
| 5.0 |
10.27
| 5.92 | 2.05 | 0.99 | 7.18 |
Role physical
| |||||||||
RP1 cut down time on work/act. | 1.74 | 0.63 | 4.20 |
10.26
NU
*
|
17.13
NU
*
| 0.44 | 0.05 | 2.65 |
9.18
NU
|
RP2 accomplished less |
10.98
U
| 1.05 | 1.43 | 0.89 | 0.72 | 0.48 | 0.44 | 2.55 | 4.14 |
RP3 limited in kind of work/activities | 1.58 | 2.08 | 1.75 | 3.62 | 0.72 | 8.57 | 0.28 | 6.54 | 0.16 |
RP4 difficulty performing work/act. | 2.10 | 1.69 | 2.80 | 1.04 |
10.52
U
| 5.42 | 0.15 | 1.26 | 0.80 |
Social functioning
| |||||||||
SF1 interferes with social activities |
14.95
NU
| 3.82 | 9.05 | 0.87 |
19.50
NU
*
| 0.43 | 1.71 | 0.74 | 1.46 |
SF2 time interferes with social act. | 9.67 | 9.57 | 1.33 | 5.95 | 7.16 | 4.80 | 1.83 | 0.22 | 4.78 |
Role emotional
| |||||||||
RE1 cut down time work/activities | 0.42 | 0.25 | 5.75 | 1.40 | 1.14 | 0.12 | 1.03 | 0.32 | 0.97 |
RE2 accomplished less | 6.68 | 0.35 | 0.18 | 2.00 | 6.45 | 4.06 | 3.46 | 5.45 |
9.32
*
|
Not as careful work/activities | 6.94 | 1.15 | 1.66 | 2.30 | 2.66 | 5.00 | 4.85 | 4.21 | 0.97 |
Mental
| |||||||||
MH1 nervous | 3.05 |
14.50
U
*
| 4.19 | 0.74 | 5.40 |
18.11
NU
*
| 1.53 | 4.24 | 2.31 |
MH2 down in dumps | 4.14 | 0.98 |
12.31
U
*
| 0.65 | 0.95 | 7.28 | 6.96 | 2.32 | 1.24 |
MH3 calm and peaceful | 8.48 | 6.54 | 1.23 | 4.00 | 6.36 |
46.33
NU
*
| 11.48 | 4.63 | 9.75 |
MH4 downhearted and low | 2.39 | 1.11 | 4.13 | 10.24 | 5.09 |
14.35
NU
*
| 8.22 | 0.02 | 1.29 |
MH5 happy person | 0.93 | 1.64 | 8.36 | 3.18 | 9.18 |
47.19
NU
*
| 3.82 | 0.27 | 2.60 |
Vitality
| |||||||||
V1 full of life | 6.93 | 0.16 | 3.25 | 7.25 | 4.56 | 7.48 | 3.09 | 4.95 | 0.28 |
V2 energy |
10.91
U
| 0.36 | 0.17 | 1.10 | 1.67 | 5.35 | 2.57 | 2.70 | 0.44 |
V3 worn out |
8.82
U
| 0.33 | 0.05 | 3.25 |
10.45
U
| 0.34 | 1.68 | 4.38 | 0.45 |
V4 tired | 5.49 | 8.19 | 2.48 | 2.20 | 7.07 | 2.59 | 0.20 | 0.91 | 0.88 |
General health
| |||||||||
GH1 health in general | 9.12 | 2.30 | 6.04 | 2.21 | 8.77 | 3.05 | 1.49 | 2.48 | 2.66 |
GH2 get ill more easily than others | 2.26 | 3.70 | 1.01 | 6.64 | 1.39 |
12.03
U
*
| 1.62 | 3.71 | 3.16 |
GH3 as healthy as anybody I know | 2.47 | 5.74 | 6.31 | 2.51 | 5.02 | 3.97 | 2.41 | 2.10 | 0.26 |
GH4 except health to get worse | 4.36 | 10.26 | 6.61 | 10.71 | 4.47 | 3.46 | 1.68 | 4.12 | 2.41 |
GH5 my health is excellent | 0.94 | 1.05 | 0.91 | 2.49 | 1.10 | 9.85 | 2.77 | 7.90 | 0.23 |
Impact
Testing for group differences using original and DIF-free measures
SF-36 sub-scale | Age | Gender | Social class | Social dep | Employment status | Mood | BMI | No joints | Hip/Knee |
---|---|---|---|---|---|---|---|---|---|
Phys
| |||||||||
Original | 4.80*** | 3.71*** | 3.82*** | 3.50*** | 8.53*** | 7.63*** | 7.17*** | 1.39 ns | 0.09 ns |
No DIF | 4.89*** | 3.24*** | 7.30*** | 7.14*** | |||||
Effect size | p = 0.015 | p < 0.0005 | p = 0.21 ns | p = 0.02 | |||||
Pain
| |||||||||
Original | -1.06 ns | 1.93* | 5.63*** | 2.99*** | 3.77*** | 6.18*** | 6.67*** | -0.23 ns | 1.35 ns |
No DIF | 5.95*** | ^2.72***/4.43*** | |||||||
Effect size | p = 0.09 | p = 0.006/0.006 | |||||||
Role Phys
| |||||||||
Original | 5.40*** | 1.70 ns | 3.27*** |
2.27*
| 8.89*** | 5.13*** | 4.22*** | 1.29 ns | 0.34 ns |
No DIF | 4.82*** |
1.81 ns
| 9.24*** | 0.16 ns | |||||
Effect | p = 0.02 |
p = 0.07
| p = 0.002 | p = 0.42 | |||||
Social
| |||||||||
Original |
1.54 ns
| 2.61*** | 4.13*** | 4.09*** | 5.37*** | 12.68*** | 4.69*** | 0.23 ns | -0.45 ns |
No DIF |
2.17*
| 4.01*** | 6.20*** | ||||||
Effect |
p = 0.04
| p = 0.37 | p = 0.002 | ||||||
Role Emot
| |||||||||
Original | 2.92*** | 1.66 ns | 3.27*** | 3.57*** | 4.81*** | 8.98*** | 3.16*** | -0.12 ns | 0.75 ns |
No DIF | 1.22 ns | ||||||||
Effect | p = 0.12 | ||||||||
Mental
| |||||||||
Original | -2.55* | 2.89*** | 3.45*** | 3.49** | 0.04 ns | 22.24*** | 2.94*** | -1.13 ns | 0.12 ns |
No DIF | 2.12* | 2.88*** | |||||||
Effect | p = 0.003 | p < 0.0005 | |||||||
Vitality
| |||||||||
Original | 0.12 ns | 2.55* | 2.31* | 3.15*** | 2.95*** | 12.29*** | 4.96*** | 0.26 ns | -0.55 ns |
No DIF | 0.14 ns | 3.53*** | |||||||
Effect | p = 0.22 | p = 0.001 | |||||||
Gen health
| |||||||||
Original | -2.56* | -0.80 ns | 4.42*** | 4.34*** | 2.42* | 11.14*** | 3.96*** | -1.59 ns | -1.25 ns |
No DIF | 10.17*** | ||||||||
Effect | p = 0.80 |