A graphical representation of all possible outcomes and the paths by which they may be reached; often used in classification tasks. The top layer consists of input nodes (e.g., meteorological observations and data). Decision nodes determine the order of progression through the graph. The leaves of the tree are all possible outcomes or classifications, while the root is the final outcome (for example, a weather prediction or climate classification). Nearly all expert systems, and many meteorological algorithms, can most appropriately be diagrammed as a decision tree. The root of the tree represents the first test, while the leaves (nodes that do not lead to further nodes) represent the set of possible conclusions or classifications. See also backward chaining, breadth-first search, depth-first search, forward chaining.