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Table 2 Overview of the statistical approach, criteria, and results of the Rasch analysis of the LFS short form scale when used with people with stroke and osteoarthritis (n = 525)

From: Psychometric properties of a short version of Lee Fatigue Scale used as a generic PROM in persons with stroke or osteoarthritis: assessment using a Rasch analysis approach

Aspect of validity evidence measured

Statistical approach and criteria

Initial LFS short form scale (5 items)

Final LFS short form scale (3 items)

Step 1:

Evidence based on internal structure

Rating scale functioning

The average measures for each response category should advance monotonically, with outfit MnSq values < 2.0 for each of the step calibrations

Criteria were met

Criteria were met

Step 2:

Evidence based on internal structure (item misfit)

Item goodness-of-fit statistics

A sample-size adjusted criterion for item goodness-of-fit requiring infit MnSq values between 0.7 and 1.3

#17 close eyes: MnSq = 1.78

#16 conversation: MnSq = 1.42

#5 worn out: MnSq = 0.75

#1 tired: MnSq = 0.67

#4 fatigued: MnSq = 0.66

#5 worn out: MnSq = 1.02

#1 tired: MnSq = 1.10

#4 fatigued: MnSq = 0.80

Step 3:

Evidence based on internal structure (unidimensionality)

Principal component analysis of the residuals

The criterion was set for at least 50% of the total variance to be explained by the first latent variable

63.2%

81.6%

Step 4:

Evidence based on response processes

Person goodness-of-fit statistics

A criterion of < 5% of the sample has unacceptable person goodness-of-fit, as indicated by infit MnSq values < 1.4 and standardized z values < 2.0.

24 (4.6%)

29 (5.6%)

Floor and ceiling effects

A criterion of up to 10% of the sample could demonstrate minimum or maximum scores.

Maximum score = None

Minimum score = 77 (14.0%)

Maximum score = 3 (0.6%)

Minimum score = 88 (16.9%)

Step 5:

Item stability

Differential item functioning (DIF)

No Mantel-Haenzel statistics have a p < .01.

Items 4 and 5 demonstrated DIF by diagnosis

No item demonstrated DIF by diagnosis

Step 6:

Person-separation reliability

Person-separation index

An index of > 2.0 was required to ensure that the scale could differentiate the sample into at least three different levels of fatigue severity

Separation index = 1.82

Separation index = 2.49

Step 7:

Internal consistency

Person reliability (Rasch)

Cronbach’s alpha coefficient (Cr-α) (raw scores)

Relationship between individual sum scores and Rasch-based measures

The criterion was that all values should be > 0.80

Person reliability (Rasch) = .77

Cr-α (raw score) = .87

Correlation between sum scores and Rasch measures = .91

Person reliability (Rasch) = .89

Cr-α (raw score) = .91

Correlations between sum scores and Rasch measures = .98

  1. MnSq Mean sq