Table 1.
Statistical tests and criteria for assessment to examine specific measurement properties of the SAQ
| Measurement property | Purpose | Statistical test | Criteria for assessment |
|---|---|---|---|
| Unidimensionality | To assess whether items in each SAQ domain measure a single construct (or concept) | • Residual fit statistics • Item-fit residuals • Person-fit residuals |
• A fit residual SD value >1.5 would suggest a problem [18]. • Item residuals that range between −2.5 and 2.5 indicate adequate fit to the model [18].a |
| Response formats (thresholds) | To assess whether participants had difficulty discriminating between the response options on the SAQ | • Threshold map • Category probability curves |
• Pattern of thresholds examined. • Ordering of thresholds where each response category systematically take turns to be the most likely response [18]. |
| Targeting | To assess whether the 6 SAQ domains are appropriately targeted for the clinical population (floor and ceiling effects) | • Mean location score • Person-item threshold distribution map |
• The mean sample location should approximate the mean item location (i.e. zero) for a well-targeted measure [18]. |
| Internal consistency reliability | To assess the extent to which items distinguish between levels of safety climate | • Person separation index (PSI) | • A PSI of a > 0.7 indicates the items of the scale is able to separate the participants in the sample [24].b |
| Item bias | To assess whether different groups within the sample (e.g. medical or surgical ward and nursing qualification), despite equal levels of the underlying characteristic being measured, responds in a different manner to an individual item | • Differential item functioning (DIF) | • Uniform DIF is indicated by a significant main effect for the person factor (e.g. ward type) [18]. • Non-uniform DIF is indicated by a significant interaction effect [18]. |
aItems with large negative residual values indicate a high level of predictability in responses and signal possible item redundancy. Items with large positive residual values suggest an item does not contribute to the measurement of a unidimensional construct
bA PSI is the same as Cronbach’s alpha with the logit value replacing the raw score in the same formulae