Schemata are employed in the process of interpreting sensory data, in retrieving information from memory, in organizing actions, in determining goals and subgoals, in allocating resources, and, generally, in guiding the flow of processing in the system. Perhaps the central function of schemata is in the construction of an interpretation of an event, object, or situation—that is, in the process of comprehension. Schemata are active computational devices capable of evaluating the quality of their own fit to the available data. Schemata consist of subschemata as procedures consist of subprocedures. A schema is said to be activated from the bottom-up whenever a subschema that has been somehow activated causes the various schemata of which it is a part to be activated. One of the central problems of a schema theory is a specification of the process whereby new schemata are developed.