Systematic Reviews and Review Types: What is a Review of Complex Interventions?

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Is your review question complex?

Review questions are harder to define when they involve complex interventions.

Definition of Complex Interventions
(adapted from Guise et al, AHRQ Series on Complex Intervention Systematic Review-Paper 1)

All complex interventions have two common characteristics:

  • they have multiple components (intervention complexity)
  • and complicated/multiple causal pathways, feedback loops, synergies, and/or mediators and moderators of effect (pathway complexity).

Additionally, they may also have one or more of the following characteristics:

  • target multiple participants, groups, or organizational levels (population complexity);
  • require multifaceted adoption, uptake, or integration strategies (implementation complexity);
  • or work in a dynamic multi-dimensional environment (contextual complexity).


Behavioral interventions for medication compliance is an example of a complex intervention.  They may “use multiple approaches, target multiple levels (individual, system, family, provider, etc.), require multifaceted training and/or adoption, and/or require special consideration about the environment/context and available resources (Guise et al. AHRQ CI series)

What is a Review of Complex Interventions? Outline of Stages

This is an outline for a systematic review of complex interventions, if you are planning to do another review of complex interventions that is not a systematic review, please also refer to the stages of that particular review type. 

Timeframe: 12 months or greater, considering the complexity of the intervention *Varies beyond the type of review. Depends on many factors such as but not limited to: resources available, the quantity and quality of the literature, and the expertise or experience of reviewers” (Grant et al. 2009)

Question: Focused PICO question, specifying the complexity of the intervention and the sources of complexity of primary interest (See Definition of Complex Interventions). A priori review protocol is recommended, however question may be expanded iteratively as complexity is explored.  May incorporate social theory or logic models to develop the question.

Review should also address the following:

(1) How effective is the intervention? (2) For whom does the intervention work and in what contexts? (3) What happens when the intervention is implemented? and (4) What decisions are possible given the results of the synthesis?

Sources and searches: Comprehensive, exhaustive search, aims to be transparent, thorough and repeatable.  Includes database, grey literature/clinical trial registry and handsearching of the literature. Must include a PRISMA flow diagram.   

Selection: Based on inclusion/exclusion criteria, may be more interative.

Appraisal: Includes a quality assessment of study bias/validity.

Synthesis: Narrative and/or meta-analysis or an alternative (see Table 1 within Tanner-Smith paper). 

(Sources: AHRQ CI series, PRISMA-CI paper, Noyes et al., Anderson et al.)

Limitations of a Review of Complex Interventions

  • To date, there is no single agreed on definition of complex interventions. 
  • May be more time consuming than a non-complex review as inputs from stakeholders and the use of theory may be necessary.
  • May require substantial adaptation of conventional review methods.
  • New methods and strategies for communicating and framing complexity is likely.

(Sources: AHRQ CI series, Noyes et al., Anderson et al.)

Examples of Reviews of Complex Interventions

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