 This model marks the beginning of inventory theory, which includes the Wagner-Within procedure, the newsvendor model, base stock model and the Fixed Time Period model.These models usually involve the calculation of cycle stocks and buffer stocks, the latter usually modeled as a function of demand variability.The economic production quantity (EPQ) differs from the EOQ model only in that it assumes a constant fill rate for the part being produced, instead of the instantaneous refilling of the EOQ model.