Economic models can use either a patient-level or cohort-level modelling approach to estimate the expected costs and outcomes across a particular population. In a patient-level simulation, the outcomes are modelled for individual patients and then the average is taken across a sufficiently large sample of patients, whilst in a cohort-level model the outcomes are estimated for the cohort as a whole without considering the outcomes for individual patients within that cohort. The optimal approach will depend on the particular nature of the decision problem being modelled and therefore needs to be assessed on a case-by-case basis.

The aims of this Technical Support Document (TSD) on patient-level simulation are to:

• Increase awareness of patient-level modelling approaches and highlight the key differences between patient-level and cohort-level modelling approaches.
• Provide guidance on situations in which patient-level modelling approaches may be preferable to cohort-level modelling approaches.
• Provide a description of how to implement a patient-level simulation illustrated with example models.
• Provide guidance on good practice when developing and reporting a patient-level simulation to inform a NICE Technology Appraisal.

This document focuses on the application of patient-level simulation to systems where patients can be assumed to be independent. However, a brief discussion of the application of patient-level simulation to situations involving patient interaction is given.