The first step in many text-as-data studies is to find documents that address a specific topic within a larger document set. Researchers often rely on simple keyword searches to do this, even though this may introduce considerable selection bias. Such bias may be even greater when researchers lack the domain knowledge required to make informed search decisions, for example, in cross-national research or research on unfamiliar social contexts. We propose expert-informed topic modeling (EITM) as a hybrid approach to tackle this problem. EITM combines the validity of external domain knowledge captured through expert surveys with probabilistic topic models to help researchers identify subsets of documents that cover initially unknown domain-specific topics, such as specific events and debates, that belong to a researcher-defined master topic. EITM is a flexible and efficient approach to the thematic selection of documents from large text corpora for further study. We benchmark and validate the method by discovering blog posts that address the public role of religion within large corpora of Australian, Swiss, and Turkish blog posts and provide researchers with a complete workflow to guide the application of EITM in their own work.
