Health-system complexity-related questions that a synthesis of quantitative and qualitative evidence could address (derived from Petticrew et al17)
| Aspect of complexity of interest | Examples of potential research question(s) that a synthesis of qualitative and quantitative evidence could address | Types 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 |
| 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 properties | What 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 loops | What 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) outcomes | What 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) |