Measuring engagement in deliberate self-harm behaviours: psychometric evaluation of six scales

BACKGROUND: Engagement in Deliberate Self-Harm (DSH) is commonly measured by behavioural scales comprised of specific methods of self-harm. However, there is a scarcity of information about the degree to which the methods relate to the same DSH construct although such scales are routinely used to provide a DSH total score. This study addresses the shortfall by evaluating the dimensionality of six commonly used behavioural measures of DSH. METHODS: The DSH measures were Self-Injury Questionnaire Treatment Related (SIQTR), Self-Injurious Thoughts and Behaviors Interview (SITBI), Deliberate Self-Harm Inventory (DSHI), Inventory of Statements About Self-Injury (ISAS), Self-Harm Information Form (SHIF) and Self-Harm Inventory (SHI). The behavioural scales contained in each measure were administered to 568 young Australians aged 18 to 30 years (62% university students, 21% mental health patients, and 17% community members). Scale quality was examined against the stringent standards for unidimensional measurement provided by the Rasch model. RESULTS: According to the stringent post-hoc tests provided by the Rasch measurement model, there is support for the unidimensionality of the items contained within each of the scales. All six scales contained items with differential item functioning, four scales contained items with local response dependency, and one item was grossly misfitting (due to a lack of discrimination). CONCLUSIONS: This study supports the use of behavioural scales to measure a DSH construct, justifies the summing of items to form a total DSH score, informs the hierarchy of DSH methods in each scale, and extends the previous evidence for reliability and external validity (as provided by test developers) to a more complete account of scale quality. Given the overall adequacy of all six scales, clinicians and researchers are recommended to select the scale that best matches their adopted definition of DSH.