Table 3 Key concepts and definitions

Adaptation

A systematically planned and proactive process of intervention modification [29, 30, 34, 35, 45, 47, 48, 51,52,53,54, 57, 62, 64] with the aim to suit the specific characteristics and needs of a new context and enhance intervention acceptability [29, 30, 34, 35, 45, 46, 48, 49, 51, 53, 54, 57, 61, 62, 64].

Mutual adaptation involves adaptation of both the intervention and of the community or organisation in which the intervention is implemented for the purposes of institutional accommodation [57, 62].

Adaptive interventions

Those interventions for which stakeholders are allowed, or even encouraged, to bring changes to the original design. These changes are pre-defined by intervention developers. In the context of complex public health interventions, involving different organisational levels and targeting collective behaviours, implementers can also make changes which are not pre-defined by the developers [31, 57].

Core components

Those features in the intent and design of an intervention which are responsible for the effectiveness of the intervention [32, 34, 36, 41, 43, 45, 48, 49, 52, 62]. Guidance suggests that these components fundamentally define the intervention [34, 43, 45, 48, 62] and therefore should not be modified in adaptation [30, 45, 48, 52], e.g. developing a natural support system for youth and families as part of a family-based intervention.

Alternative terms: essential, necessary, prototypical components or elements, or intervention’s deep structure.

Discretionary components

Those features which are not essential for the target audience and which are not supported by the theory of change and thus are assumed to be modifiable without major impact on intervention effectiveness [45, 52, 58, 62], e.g. provision of an additional class as part of a parenting intervention addressing trauma related to natural disasters.

Alternative terms: optional components or intervention’s surface structure.

Drift

A misapplication or a mistaken application of an intervention involving technical errors, abandonment of core components, or introduction of counterproductive elements resulting in a loss of intervention benefits [29, 54].

Fidelity (adherence)

The degree to which an intervention is implemented as intended by its developers [29, 47,48,49, 54, 58, 59, 62] with the aim to maintain intervention’s intended effects [57, 58]. The components of fidelity (also dimensions for measuring fidelity) include dose, frequency, exposure, quality of delivery, participant responsiveness, and programme differentiation [29, 49, 57, 59, 62].

Programme theory

Refers to the causal model that specifies the empirical and theoretical relations between intervention activities, mediators of change, and ultimate outcomes [34, 43, 45, 58].

Alternative terms: theory of change, internal logic

Reinvention

The degree to which an innovation (i.e. an intervention) is changed or modified by the user in the process of its adoption and implementation [37, 57, 62].

Replication

The process of re-implementing an established intervention in a new context in a way that maintains fidelity to core goals, activities, delivery techniques, intensity, and duration of the original study [34].

Transcreation

The processes of planning and delivering interventions so that they resonate with the targeted community, while achieving intended health outcomes [47].

Scale-out

The deliberate use of strategies to implement, test, improve, and sustain an intervention as it is delivered to new populations and/or through new delivery systems that differ from those in effectiveness trials. Aarons et al. distinguish three types of scale-out: type I scale-out: population fixed, different delivery system; type II scale-out: delivery system fixed, different population; type III scale-out: different population and delivery system [30].

Scale-up

The deliberate effort to broaden the delivery of an intervention with the intention of reaching larger numbers of a target audience. It often targets the same or very similar settings, under which the intervention has already been tested [30].

Social validity

Refers to perceived acceptability, utility, and viability of the intervention. These perceptions might be influenced by cultural worldview and the practical realities of life circumstances (e.g. transportation, insurance coverage, and work schedules) [31, 42].