Who is the model for?
It is necessary to identify the audience of the model at an early stage. Understanding the audience, and their interests can inform the type of model you build and the results that the model should generate. The audience could be as broad as the healthcare providers of a country of choice, or just an individual hospital or clinic.
What is the perspective of the model?
Models include different information according to the perspective that has been assumed. For example, a model with a healthcare provider perspective (e.g. NHS) will only consider costs that are paid for by the NHS; a model with a societal perspective would also include costs such as out-of-pocket costs paid for by the patient and the cost to society of the lost productivity due to the individual feeling ill at work.
What comparisons should the model make (if any)?
The model may attempt to make comparisons between treatment options, cost-effectiveness between disease subgroups or healthcare strategies. All relevant comparators should be identified from the outset, so that the model can be built to make such comparisons possible. It is also necessary to determine what type of comparisons are being made, and whether the focus is on costs, health benefits, or both; this will inform the choice of model (e.g. cost-effectiveness model, COI model or BIM).
What is the time horizon of the model?
The results that the model calculates will be for a specific time period, called the time horizon. It is necessary to make sure that costs and/or benefits are only included for the time period of choice. Depending on the nature of the condition and data availability, the time horizon may extend to the entirety of a patient’s lifetime. Alternatively, the time horizon may be selected to be shorter, either as a simplification or because the disease or condition is only transient. As time horizons expand, it is important to remember that there is likely to be increasing uncertainty, for example, costs may fluctuate unpredictably due to unforeseen external influences.
What is the level of complexity required?
It is important that an economic model includes sufficient detail to be representative of reality but remains simple enough to be built in the available time frame and to be easily understood.
What sources are available?
The model will include a number of inputs relating to the patient population, benefits of the treatment and any comparators, and costs. It is important to consider what data are available before making a decision about the type of model.
There may be information in the literature or results from other health economics models, which could be useful for sourcing inputs or information on any considered comparators. In the United Kingdom, unit costs of health and social care are found in cost databases such as the NHS reference costs, and developed by the Personal and Social Services Research Unit (PSSRU).
Often in the absence of suitable data it may be necessary to seek guidance from clinical experts who may be able to provide informed estimates, based on their experience. If it is not possible to gather expert opinion, assumptions may need to be made, which can be tested in different scenarios.