含固定訂購成本變動期間定期盤存模式之研究

Abstract

本計畫探討含固定訂購成本之變動期間定期盤存制。定期盤存制在實務上廣泛使 用，特別是當許多商品向同一供應商購買時更是如此。但幾乎所有定期盤存模式皆假設 盤存期間為固定，而實務上盤存期間卻可能為變動。此現象主要肇因於供應鏈中之供應 不確定，如供應商拜訪下游零售商並補充存貨，但並非固定間隔時間到達。變動期間也 可能因組織內部不確定所造成，如臺灣醫院藥局常每隔兩周實施循環盤點並隨即補貨， 但或因盤點所需人力不足，或因協調訂購，藥局經常未能以固定間隔時間採購。 本計畫延伸姜齊2008 年國科會之研究，使用動態規劃來處理上述變動期間定期盤 存制，且加入固定訂購成本。本計畫假設盤存期間為獨立且同質分配，初步研究結果顯 示所導出之存貨模式為文獻上標準之定期盤存問題，因此極小化每期成本或使用熟知的 (s, S)模式可得到最佳解。本計畫所發展之存貨政策易於為業界所應用。 另初步計算結果顯示，若固定訂購成本不大，忽略變動盤存期間將導致巨大的損 失，因此必須採用本計畫建議之存貨模式。但當固定訂購成本大時，則可使用一般定期 盤存模式。

Periodic review inventory models are widely used in practice, especially for inventory systems in which many different items are purchased from the same supplier. However, most periodic review models have assumed a fixed length of the review periods. In practice, it is possible that the review periods are of a variable length. Such periodic systems result mainly from supply (external) uncertainties. For example, the supplier visits the downstream retailers and replenishes inventories for them, but does not always come in constant intervals. Stochastic review periods may also arise from organizational (internal) uncertainties. For example, it is common in Taiwan that the hospital pharmacy performs cycle counting every two weeks and replenishes the inventory of various items afterwards. However, due to the availability of manpower needed for cycle counting and/or coordination and consolidation of ordering, the pharmacy often does not order in fixed-length time intervals. In this research, we extend the work of Chiang [International Journal of Production Economics, 115 (2008) 433-438], and consider periodic review models where the review periods are of a variable length and there is a fixed cost of ordering for replenishment. Assuming that period lengths are independently and identically distributed, we show (using an exact method of computing inventory holding costs) that an order-up-to or (s, S) policy is optimal for the infinite horizon problem. The periodic review policies developed are thus easy to implement. Preliminary computational results show that if the fixed cost of ordering is small, one needs to use the proposed periodic policies. However, when the fixed ordering cost is large, it is probably alright to use the ordinary periodic review models.