Table 3 Interactions and interdependencies within complex interventions (adapted from [40, 41])

Complex interventions can include components that interact synergistically or dysynergistically, as follows:

• Synergistic: Intervention components interact in ways that the total effect is greater than the sum of the individual effects of the components.

• Dysynergistic: Intervention components act in ways that the total effect is less than the sum of the individual effects of the components.

Where intervention components do not interact in these ways, one would expect the effect of the intervention to be the sum of the individual effects of all of the components.

Complex interventions can also include components that are interdependent. Where such interdependencies exist, they can be described as:

• Contemporaneous: The effect of one intervention component depends on another intervention component being present at the same time. On their own, each component may be less effective, ineffective, or harmful.

• Temporal: The effect of one intervention component depends on another component being present beforehand. On their own, each component may be less effective, ineffective, or harmful.

Where intervention components do not show interdependency, one would expect these components to be effective regardless of the presence or absence of other components.