A considerable amount of research has been made on the safety, reliability and validity of the Functional Capacity Evaluation (FCE). Evidence is accumulating on different aspects of validity of FCE in healthy workers and in different patient groups, which has indicated that we are measuring a complex construct consisting of biological, psychological and social variables. However, this knowledge is still unstable as the majority of studies stem from a limited amount of countries. The present project focuses on the biopsychosocial factors that influence FCE performance. Participants undergoing an FCE perform differently depending on biological, psychological and social variables, although the latter has been under-researched. Clinicians are currently unable to predict which factors determine the differences in patient's FCE results. In this project, in addition to biological and psychological variables, investigators include a variety of social variables from different countries, jurisdictions, and evaluators, which may help investigators to identify important determinants of FCE performance. Knowing which characteristics and behaviors are associated with better or poorer participant performances may help clinicians to deliver improved FCE services.