This drop-down menu option limits your search to a dataset you select. Datasets are an outcome of research projects. Sometimes these projects are discrete (and therefore the results do not alter) and sometimes they are ongoing (so the results of searches will change over time). The Mitchell Treasures dataset is small and complete, for example, while the Multicultural Writers dataset is vast and ever-growing.
Check the pages under the 'Research' tab for project information to help you understand what to expect when you deploy this option in your search. Some limit to people and works but some limit to only people or only works.
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