Two-level theories explain outcomes with causal variables at two levels of analysis that are systematically related to one another. Although many prominent scholars in the field of comparative analysis have developed two-level theories, the empirical and methodological issues that these theories raise have yet to be investigated. In this article, the authors explore different structures of two-level theories and consider the issues involved in testing these theories with fuzzy-set methods. They show that grasping the overall structure of two-level theories requires both specifying the particular type of relationship that exists between and within levels of analysis and specifying the logical linkages between levels in terms of necessary and sufficient conditions. They argue that for the purposes of testing these theories, fuzzy-set analysis provides a powerful set of tools. However, to realize this potential, investigators using fuzzy-set methods must be clear about the two-level structure of their theories from the onset.
