Skip to main content

Table 9 Results of studies assessing structural validity of the instruments included in the review

From: Measuring carer quality of life in Duchenne muscular dystrophy: a systematic review of the reliability and validity of self-report instruments using COSMIN



Mean Age (SD)

% female


Analysis model

Key result(s)

Rating of measurement property


Quality of evidence

ZBI (22 item) [60]


44 (NR)



Rasch partial credit model

"In total, nine of 22 items (41%) displayed model misfit in terms of estimated residuals, all but two at a significant χ2 probability. Four misfitting items had a large negative residual, suggesting that these may not add any new information to the scale. The overall item–trait interaction chi-square value was 499, 198 degrees of freedom, p < 0.001, indicating that the items were not working as expected across different levels (i.e., class intervals) of burden (…) Mean item dependency was low (0.042) (…) Disordered thresholds were identified for 13 of 22 items (59%) (…) Minimal floor effect (< 1%, 1 of 475) and no ceiling effect (…) Fit residuals ranged from 0.02 to 4.76 (9 items exhibited misfit) (…) Taken together, results from our analysis showed that the English (UK and US) version of ZBI may not be regarded as a unidimensional, interval rating scale of burden among caregivers to patients with DMD.”


  1. Citation next to the instrument is for the study assessing this measurement property
  2. NR, not reported