The Comparative Data Analysis Ontology (CDAO) provides a framework for understanding data in the context of evolutionary-comparative analysis. This comparative approach is used commonly in bioinformatics and other areas of biology to draw inferences from a comparison of differently evolved versions of something, such as differently evolved versions of a protein. In this kind of analysis, the things-to-be-compared typically are classes called 'OTUs' (Operational Taxonomic Units). The OTUs can represent biological species, but also may be drawn from higher or lower in a biological hierarchy, anywhere from molecules to communities. The features to be compared among OTUs are rendered in an entity-attribute-value model sometimes referred to as the 'character-state data model'. For a given character, such as 'beak length', each OTU has a state, such as 'short' or 'long'. The differences between states are understood to emerge by a historical process of evolutionary transitions in state, represented by a model (or rules) of transitions along with a phylogenetic tree. CDAO provides the framework for representing OTUs, trees, transformations, and characters. The representation of characters and transformations may depend on imported ontologies for a specific type of character.