Comparison is a valuable and widely touted analytical technique in social research, but different disciplines and fields have markedly different notions of comparison. There are at least two important logics for comparison. The first, the logic of juxtaposition, is guided by a neopositivist orientation. It uses a regularity theory of causation; it structures the study by defining cases, variables, and units of analysis a priori; and it decontextualizes knowledge. The second, the logic of tracing, engages a realist theory of causation and examines how processes unfold, influenced by actors and the meanings they make, over time, in different locations, and at different scales. These two logics of comparison lead to distinct methodological techniques. However, with either logic of comparison, three dangers merit attention: decontextualization, commensurability, and ethnocentrism. One promising research heuristic that attends to different logics of comparison while avoiding these dangers is the comparative case study (CCS) approach. CCS entails three axes of comparison. The horizontal axis encourages comparison of how similar policies and practices unfold across sites at roughly the same level or scale, for example across a set of schools or across home, school, religious institution, and community organization. The vertical axis urges comparison across micro-, meso-, and macro-levels or scales. For example, a study of bilingual education in the United States should attend not only to homes, communities, classroom, and school dynamics (the micro-level), but also to meso-level district, state, and federal policies, as well as to factors influencing international mobility at the macro-level. Finally, the transversal axis, which emphasizes change over time, urges scholars to situate historically the processes or relations under consideration.
