Table 2

Health-system complexity-related questions that a synthesis of quantitative and qualitative evidence could address (derived from Petticrew et al 17)

Aspect of complexity of interestExamples of potential research question(s) that a synthesis of qualitative and quantitative evidence could addressTypes of studies or data that could contribute to a review of qualitative and quantitative evidence
What ‘is’ the system? How can it be described?What are the main influences on the health problem? How are they created and maintained? How do these influences interconnect? Where might one intervene in the system?Quantitative: previous systematic reviews of the causes of the problem); epidemiological studies (eg, cohort studies examining risk factors of obesity); network analysis studies showing the nature of social and other systems
Qualitative data: theoretical papers; policy documents
Interactions of interventions with context and adaptation
  1. For a research question about Implementation: (How and why) does the implementation of this intervention vary across contexts?

  2. For an effectiveness review: Do the effects of the intervention appear to be context dependent?

Qualitative: (1) eg, qualitative studies; case studies
Quantitative: (2) trials or other effectiveness studies from different contexts; multicentre trials, with stratified reporting of findings; other quantitative studies that provide evidence of moderating effects of context
System adaptivity (how does the system change?)(How) does the system change when the intervention is introduced? Which aspects of the system are affected? Does this potentiate or dampen its effects?Quantitative: longitudinal data; possibly historical data; effectiveness studies providing evidence of differential effects across different contexts; system modelling (eg, agent-based modelling)
Qualitative: qualitative studies; case studies
Emergent propertiesWhat are the effects (anticipated and unanticipated) which follow from this system change?Quantitative: prospective quantitative evaluations; retrospective studies (eg, case–control studies, surveys) may also help identify less common effects; dose–response evaluations of impacts at aggregate level in individual studies or across studies included with systematic reviews (see suggested examples)
Qualitative: qualitative studies
Positive (reinforcing) and negative (balancing) feedback loopsWhat explains change in the effectiveness of the intervention over time?
Are the effects of an intervention are damped/suppressed by other aspects of the system (eg, contextual influences?)
Quantitative: studies of moderators of effectiveness; long-term longitudinal studies
Qualitative: studies of factors that enable or inhibit implementation of interventions
Multiple (health and non-health) outcomesWhat changes in processes and outcomes follow the introduction of this system change? At what levels in the system are they experienced?Quantitative: studies tracking change in the system over time
Qualitative: studies exploring effects of the change in individuals, families, communities (including equity considerations and factors that affect engagement and participation in change)