Complexity among people experiencing homelessness: A scoping review of definitions and measurement approaches.

People experiencing homelessness are often described as having "complex needs" for whom various health and housing interventions have been developed. However, there is no consensus on how this and similar terms are defined or measured. A prospectively registered scoping review was conducted to examine how "complexity" is defined and measured with people experiencing homelessness. Six academic databases were searched, along with several techniques for identifying grey literature and articles in unindexed journals. A total of 224 academic articles were included in the review (35 grey literature articles were included in a supplemental review). There were 220 definitions of complexity provided in 203 academic articles, which most commonly included mental illness and/or substance use problems. Physical health problems, multiple health conditions, justice system involvement and offending behaviours, health and social service use patterns, and type and characteristics of homelessness were also common. Three types of approaches for measuring complexity were identified in 29 articles: (1) instruments (primarily vulnerability indices and service prioritization tools), (2) risk adjustment/health resource prediction indices, and (3) study-specific algorithms and processes. Limited validation was common among the methodological approaches, with a few exceptions. Overall, the findings highlight how complexity is often underdefined, with limitations in its measurement.