The process of determining how well the outputs of a mathematical model of a real-world problem conform to reality. Two key aspects of validity are face validity and predictive validity. Face validity is based on an examination of the assumptions and data going into the model for logical consistency and the review of the results by experts knowledgeable in the real world situation. Predictive validity is based on examining the model's predictions for events that were not used in building the model. Verification; Verification, validation and testing of models.