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Table 1 Overview of the Rasch analytic process

From: Rasch analysis of the long-term conditions questionnaire (LTCQ) and development of a short-form (LTCQ-8)

Steps Psychometric property Aim Criterion
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 1st 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