An economic evaluation should consider all available evidence relevant to the decision problem.

Importance to decision making
Failure to draw on all relevant and available evidence when undertaking an economic evaluation will potentially introduce bias of unknown direction, limiting the capacity of the economic evaluation to inform a “good” decision.

Evidence can be broadly defined as factual material necessary to make an informed decision, and encompasses any information that will be used to qualitatively or quantitatively inform the design, results and conclusions of an economic evaluation. Evidence will be used to inform all elements of an economic evaluation, including the unbiased estimate of the mean clinical effectiveness and the costs and resource use of the interventions being compared.

Judgement may however be necessary as to what constitutes ‘all relevant and available evidence’. The researcher must apply this judgement in a systematic and transparent way when designing the economic evaluation in order to minimise bias. The decision-maker should also assess whether an economic evaluation contains all relevant and available evidence when deciding if it is applicable to the decision problem.

Methodology
The approach used to consider the relevance and applicability of available evidence to the decision problem should be determined before evidence gathering begins.

This methodological specification does require all economic evaluations to ensure that all parameters are informed by all available and relevant evidence, where the judgements made to determine availability and relevance should be made in a transparent and systematic way. It is not intended that this methodological specification would prevent researchers conducting single-study or within-trial based economic evaluations, as many aspects of the decision problem may be limited to a specific trial of an intervention in a particular context and time. Researchers conducting single-study or trial based analyses are still required to outline how the single study or trial considers all available evidence relevant to the stated decision problem.

While the budget and time available for the study are relevant in determining the feasibility of the economic evaluation and scope of the decision problem, these should not influence any determination of the scope of the relevant evidence. That said, while it is important that a systematic review of the literature is undertaken to obtain estimates of the clinical effects of the intervention and its comparator(s), for some other parameters the collection and synthesis of all information may be prohibitively expensive or time-consuming. In these instances a transparent judgement should be made about the likely implications of not including missing information in the economic evaluation. Where feasible, researchers should explore the implications of alternative judgments about the quality and relevance of evidence (eg. disease natural history or progression and treatment effects). This could include presenting different scenarios that represent different judgements about which evidence ought to be included. The justification for each should be clearly expressed so its plausibility can be properly considered. This should draw on the application of accepted principles of clinical epidemiology to the available studies, indications of inconsistency within the analysis, knowledge of the natural history of disease, and reference to other external evidence where appropriate.

Researchers should clearly state when the evidence available to inform aspects of the economic evaluation is weak or unavailable. This allows the decision-maker to make a judgement on the acceptability of the evidence in informing the decision (see section 2.9 on uncertainty).