To monitor the fidelity of an embedded and potentially complex clinical intervention, it is important to identify in advance which features are so essential to its effectiveness that modifying them could negatively affect the study’s outcomes and impact. Equally important is knowing which intervention elements can be adapted to accommodate contextual factors or local needs—without affecting fidelity. Another way of describing this is through the concepts of functions and forms:

• Functions are the fundamental purpose or desired effect of the intervention activities, such as the effect on patient or provider behavior.
• Forms are the intervention details, components, and activities that may vary, or be adapted, based on contextual factors.

“Functions of a complex intervention represent a purpose or goal, while forms are the tools or processes used to achieve a function. Identifying forms and functions allows adapted complex interventions to retain a level of standardization and integrity in design.” (Hill et al. 2020)

Identifying an intervention’s core functions informs how fidelity is monitored (i.e., by observing fidelity to the core functions) and what adaptations can be made without compromising effectiveness. Such fidelity is critical to demonstrating that the same intervention is being studied across sites and settings (Esmail et al. 2020). For example, take a cognitive behavioral therapy (CBT) intervention whose core function is to train patients to use coping skills. Its corresponding forms are the activities used to achieve the goal of training patients in coping skills, and might include the specific content, number, and length of intervention sessions, and the mode of delivery (e.g., group, individual, online, in-person). Moving from in-person delivery of the CBT intervention to online delivery, for example, due to the COVID-19 pandemic, would represent an adaptation to the intervention’s form—but not to the core function of the intervention.

In this section, we introduce recently developed methodology standards around intervention fidelity and adaptations for study teams to consider.

PCORI Standards for Complex Health Interventions

The Patient-Centered Outcomes Research Institute (PCORI) has synthesized a set of methodology standards to help investigators ensure the “validity, transparency, and reproducibility of studies of complex health interventions” (Esmail et al. 2020). PCORI’s conceptual model intends to help investigators evaluate the aspects of their study that go beyond the intervention’s content and delivery (i.e., the intervention components) toward a deeper delineation of how the intervention achieves its effects, referred to as the “causal pathway.” The standards require that an intervention’s core functions and forms, causal mechanisms, and expected adaptations are described during the design phase and evaluated throughout implementation, interpretation, and reporting.

Employing the PCORI methodology standards can assist study teams as they prepare to monitor fidelity and plan for adaptations or alternative means that will achieve the desired outcomes. Study teams are encouraged to refer to the publication (Esmail et al. 2020) and the PCORI methodology standards website for details on how to apply these standards to your intervention, which consist of these broad steps:

1. Fully describe the intervention and comparator and define their core functions.
2. Specify the hypothesized causal pathways* through which the core functions influence outcomes.
3. Specify how adaptations to the form of the intervention and comparator will be allowed and recorded.
4. Plan and describe a process evaluation.
5. Select patient outcomes informed by the causal pathway.

*The causal pathway is an assumption about how the intervention works or produces change (Moore et al. 2015).

“By emphasizing fidelity to core function rather than to form, [the PCORI methodology] standards aim to link the underlying purposes or goals of an intervention (rather than its content or activities) to their role in the causal pathway.” (Esmail et al. 2020)

Applying Functions and Forms to Interventions

Read more about how delineating an intervention’s functions and forms can help to advance future implementation:

• The PRIM-ER NIH Collaboratory Trial applied functions and forms to their complex intervention in Primary Palliative Care for Emergency Medicine (PRIM-ER): Applying form and function to a theory-based complex intervention (Hill et al. 2020).
• A synthesis evaluating a coordinated care model aligns with the PCORI methodology to support “an explicit theory-based depiction of how an intervention achieves its effects, links needs and contextual factors to outcomes, and illustrates how variations in form tie to core functions” in Core Functions and Forms of Complex Health Interventions: a Patient-Centered Medical Home Illustration (Perez Jolles et al. 2019).

ASPE Resource

In 2013, the Office of the Assistant Secretary for Planning and Evaluation (ASPE) of the Department of Health and Human Services developed a research brief focused on “the importance of identifying, operationalizing, and implementing the core components of evidence-based and evidence-informed interventions that likely are critical to producing positive outcomes” (Blase and Fixsen 2013). The brief, Core Intervention Components: Identifying and Operationalizing What Makes Programs Work, serves as an introduction for study teams as they identify, validate, operationalize, and adapt their program’s core components before and during study implementation.

The brief suggests that, in order for an evidence-based intervention to be useful in a real-world setting, the following elements should be present:

1. A clear description of the context of the program, the principles underlying the intervention, and the population it is intended to serve
2. A definition of the essential functions of the intervention judged as necessary to produce outcomes in a typical setting
3. A description of the active ingredients that operationally define the core components; ie, the teachable and learnable activities for providers delivering the intervention
4. Plans for a practical assessment of the performance of those delivering the intervention so that these measures can be correlated with outcomes over time

Well-defined and measured core intervention components can lead to improved use of staff resources, greater likelihood of accurately interpreting outcomes, increased ability to make appropriate adaptations, and better replication and scale-up of the intervention in other settings.