Construction of composite scales for risk assessment in epidemiology: an application to ectopic pregnancy.

Composite scoring systems that combine information contained in a number of risk factors are being used increasingly in clinical practice, planning, and health risk appraisal. The authors propose a methodological framework for the construction and validation of a composite measurement scale to assess the risk, considered as a continuous phenomenon, of developing a particular disease or outcome. This framework integrates several statistical methods, especially those concerning model fitting, coefficient rounding, and validation strategy. It also uses psychometric methods, addressing important measurement properties such as measurement level, content and construct validity, and reliability of the constructed scale. The proposed framework is illustrated by application to the construction of a composite scale for measurement of the risk of ectopic pregnancy.