AbstractIdeas and ideational change are difficult to grasp. When exactly did the idea of sustainability emerge? How has the meaning of democracy changed over time? Did ideational change precede or follow institutional change? These and many similar questions are highly relevant to the emerging field of ideational institutionalism, but they are difficult to answer conclusively without reliable data. Google Books Ngram Viewer, a tool introduced in 2011, opens up new possibilities in this regard by allowing the analysis of millions of print sources over time. However, after initial euphoria, its use has waned somewhat due to growing doubts about its reliability and the validity of the results it produces. Some of these problems have been addressed, but the debate about the potential and limitations of Google Books Ngram Viewer is ongoing. This research note will contribute to this debate by proposing a systematization for the use of search terms based on the concept of belief systems and by presenting the results of a reliability test for the German language corpus.