What must combined studies being assessed in a meta-analysis be similar in?

In a meta-analysis, it is crucial for the combined studies to be similar in population, interventions, and outcome measures because these factors directly impact the validity and reliability of the overall findings. When the populations differ significantly, variations in demographics, health status, or other variables can lead to differences in how interventions work, thus skewing the results. Similarly, if the interventions used in the studies are not comparable (e.g., different dosages, types, or modalities), the effectiveness of those interventions cannot be accurately assessed across studies. Lastly, consistent outcome measures are essential to ensure that the studies are evaluating the same endpoints, allowing for a meaningful comparison of results. Overall, these similarities help to minimize heterogeneity and ensure that the meta-analysis provides a comprehensive and accurate synthesis of the evidence.