Table 1 Overview of the Rasch analytic process

1

Rating scale function

Assess the scale’s functionality, i.e. do the category measures on each item advance monotonically

Goodness-of-fit: < 2.0 outfit MNSQ, minimum 10 participants per value per item

2

Internal scale validity

Examine how well the item responses match the expected responses in the Rasch model

Item goodness-of-fit: ≤ 1.2 MNSQ, worst fitting item removed one at a time and model subsequently re-run

3

Dimensionality

Assess if the scale measures a single construct

 > 50% total variance explained by 1^st component (Rasch model), additional components ≤ 5% (or eigenvalue ≤ 2.0) after removal of first component. No more than 1 out of 20 (or 5%) of the residual correlations > 0.30

4

Reliability

Person-separation validity: Assess if the scale can discriminate participants’ responses into groups based on performance;

Internal consistency: Assess if the item responses are consistent

Person-separation index: ≥ 2.0

Internal consistency: Cronbach’s alpha ≥ 0.80

5

Differential item functioning (DIF)

Examine how the scale functions among various groups (number of LTCs, age, gender, cohort, hospital admissions)

DIF contrast < 0.43 logits: p > 0.01