Table 1 Statistical tests and criteria for assessment to examine specific measurement properties of the SAQ

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].