在傳統EOQ模式中，為簡化模式因此設定了許多不合實際的假設，如零售商必須在商品到達時立即付款。然而在實務中，供應商可能為了增加銷貨量，通常為鼓勵零售商的訂貨意願，當訂貨量達到設定的標準數量就給予信用交易期限之優惠；若訂購量低於標準，則須在貨到時立即付款給供應商。信用交易期限優惠的有無取決於訂購量大小。另一不合理設定為假設所有生產或採購的商品皆為良品，但因製程能力與材料的限制，現實世界中100%良品的情況是相當不易達成的。本研究主要修改上述不合實際的假設，探討不良品與在大批量才允許延後付款的假設條件來發展存貨策略。
本篇論文以零售商的觀點，探討不良品項在大批量才允許延後付款的條件下之最佳存貨補充策略，建立各存貨模型的目標函數，再根據目標函數的特性，提供有效且精確的存貨補充策略。

The classical EOQ model is a simplistic model, most assumptions of the EOQ model are not unrealistic. An unrealistic assumption of the EOQ model is that the retailer must pay for the items as soon as the items are received. However, in practice, a supplier may offer a retailer a delay in payments to encourage retailer to order large quantity. When the ordering quantity is more than the fixed quantity at which the delay in payments is permitted, the fixed trade credit period is permitted. Otherwise, the payments for the items must be made immediately. The credit period depends on the quantity ordered. Another unrealistic assumption is that all units produced or purchased are of good quality. Because the bounds of process capability and material distinction, it is difficult to produce or purchase items with 100% good quality under real-life situations. This article considers above two unrealistic assumptions of classical EOQ model to emerge discussion between the imperfect items and permissible delay in payment depending on the ordering quantity to develop inventory policies.
From the viewpoint of retailer, the propose of this paper is to explore the optimal inventory policies of imperfect items under permissible delay in payments depending on the ordering quantity. We develop models to determine the optimal cycle and show the object functions that possess some properties. With those properties, the efficient and correct solution procedures are presented to determine the optimal inventory replenishment policies.

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