Table 5. Core elements for the feasibility phase.

Core elements	Issues to consider	Risk of not considering them
Context	Determine the contextual dependencies for the intervention (e.g. factors affecting delivery and receipt) and evaluation (e.g. factors affecting collection of outcome data) Consider how these contextual dependencies may change for a future evaluation or implementation within different settings Incorporate aspects of process evaluation	An intervention and/or evaluation that does not demonstrate feasibility and/or acceptability, with no understanding of why An intervention and/or evaluation that fails to meet progression criteria
Programme theory	Begin with a proposed programme theory Use programme theory to identify measurable outcomes Refine the programme theory as data are gathered to inform how the intervention operates Incorporate aspects of process evaluation to explore potential mechanisms of impact Produce a revised programme theory by the end of the phase	Key contextual dependencies are neglected resulting in an intervention that fails to operate as designed
Stakeholders	Input is critical to ensure that relevant data are collected to inform future delivery, evaluation, implementation and impact Input is critical to programme theory and key uncertainties	Lack of useful and convincing data to inform progression to the next phase of evaluation and/or implementation Planning and progression to the next phase will not meet stakeholders’needs or benefit from their critical insights Risk of proceeding with a full-scale definitive trial for an intervention that is highly unlikely to be implemented in practice
Uncertainty	What are the key questions that need to be addressed to progress to an evaluation study? Translate these into progression criteria and use these progression criteria as the principal questions to be addressed in the feasibility study Prior to evaluation, the feasibility study may assess the acceptability of intervention and evaluation to participants, providers and the research setting Consider undertaking an evaluability assessment to determine whether or not and how an evaluation should be undertaken	Progression to a large-scale, potentially expensive evaluation study that then fails because the evaluation design and methods and/or the intervention design and implementation have not been sufficiently tested for feasibility for acceptability Contextual and implementation factors undermine the intervention in ways that were not experienced or anticipated in prior development or efficacy research
Intervention refinement	Agree boundaries and limits on how much the intervention or evaluation design can be refined Report all refinement transparently	Risk of proceeding to the next phase of evaluation with a suboptimal intervention and/or evaluation design
Economic considerations	Identify and measure data on resource use across a range of cost categories consistent with the perspective of the study Identify relevant unit cost sources for the valuation of resource use Consider logistical issues with collecting resource use data (e.g. practical and logical) Develop economic data collection tools and systems. Test the acceptability of data collection tools Consider whether or not qualitative and process evaluation can feed valuable information into the economic evaluation alongside the feasibility study or economic model Identify and measure suitable economic outcomes (preferably preference based) Consider which economic evaluation frameworks are suited to the research question (possibly consider using more than one framework) Consider the use of VOI: potentially useful for identifying areas of particular uncertainty and further research, and identifying a monetary value limit for the costs of any further research activities	Underestimation of time and resources required to collect resource use and outcome data: could ultimately lead to ‘failure’of the trial or ‘ineffective intervention’owing to poor data collection Data collection tools or systems that are unacceptable to patients/clients or those delivering the intervention Inappropriately designed economic evaluation for a definitive trial (e.g. full range of resources and outcomes not correctly identified and measured) Limited economic evaluation frameworks considered Expense of proceeding with a full-scale definitive trial for an intervention that is highly unlikely to be cost-effective or implementable in practice