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Non-randomised studies of interventions (NRSIs) are defined in the Cochrane Handbook for Systematic Reviews of Interventions as any quantitative study estimating the effects (benefit or harm) of an intervention that does not use randomisation to allocate individuals (or other units such as groups of individuals) to intervention groups.1 NRSIs use a variety of designs and methods to estimate and quantify the causal effect on the outcome(s) of interest of a healthcare intervention, programme or policy (box 1). It is increasingly recognised that NRSIs have an important role to play in systematic reviews of healthcare interventions (eg,2) including on topics where interventions are typically directed to a population, such as in the areas of public health and planetary health policy, or where the effect of interest sought is of an intervention delivered in real-world settings,3 as well as in assessing harms and long-term effects of clinical practice interventions.