Table 3.

Psychometric properties of COPD and Asthma Sleep Impact Scale

Conceptual and measurement model
Rationale for deriving scale scores	Items generated from focus group discussions in UK and US samples
Scale structure	15 item scale scored 1= never to 5= very often – transferred onto a 0–100 scale
Variability	Mean score COPD patients (n=112) 47.1±24.0
Reliability
Inter-intra observer repeatability	Not tested
Item correlations	9 items highly correlated r>0.75; 6 items indicating item redundancy
Internal consistency	Cronbach’s alpha 0.91
Stability over time	2-week test-retest repeatability ICC 0.84
Validity
Convergent validity	Correlated with SGRQ r=0.68 P=0.0001 Correlations between CASIS scores and number of bad days r=0.61, overall health status (0.5), and higher mean CASIS scores in COPD patients receiving oxygen treatment (51.4 vs 43.3) Correlates with living with COPD questionnaire 0.58
Responsiveness to change	Not tested

Abbreviations: COPD, chronic obstructive pulmonary disease; CASIS, COPD and Asthma Sleep Impact Scale; ICC, intra-class correlation coefficient; SGRQ, St George’s Respiratory Questionnaire.