An ancillary study is a secondary research project conducted in parallel with a primary study, often with a different focus or objective. The primary study can be a clinical trial, an observational study, or another type of research that generates data of interest to other researchers or stakeholders.

Ancillary studies are usually conducted by other researchers who use the same data collected during the primary study. They may focus on different research questions or employ different methods of analysis. Ancillary studies are often designed to explore additional research questions that were not addressed in the primary study or to investigate specific subgroups within the study population.

The results of ancillary studies can provide valuable insights into a broader range of research questions, and can contribute to the overall understanding of the primary study. They can also help identify new areas of research and inform the development of future studies.