Cases with multiple defects can be difficult to diagnose because the defects can interact, meaning that the observable cues are not a sum of the cues for the component defects. Diagnostic methods that use cue-to-defect relationships fail when interactions between defects change the observable cues. The primary alternative, model-based methods, are limited to domains with accurate and complete models, along with initialization data. Using these traditional methods, when defects interact and models aren’t available, each possible defect combination must be included in the knowledge base. This results in an explosion of possible alternatives, greatly increased knowledge acquisition effort, slower processing, and increased maintenance effort.