📚 Worksheet – Chapter 20: Inventory Management | 章节练习 – 第20章：库存管理
Effective inventory management is crucial for any business that holds stock, as it directly impacts costs, cash flow and customer satisfaction. This worksheet reviews the key concepts from Chapter 20, including the reasons for holding inventory, inventory costs, stock control charts, just-in-time (JIT) systems and performance ratios. Use these notes to consolidate your understanding and prepare for exam-style questions.
有效的库存管理对任何持有存货的企业都至关重要，因为它直接影响成本、现金流和客户满意度。本练习回顾第20章的核心概念，包括持有库存的原因、库存成本、库存控制图、准时制（JIT）系统以及绩效比率。请使用这些笔记巩固理解，并为考试类题目做好准备。
1. What is Inventory? | 什么是库存？
Inventory (also called stock) refers to the materials, work-in-progress and finished goods that a business holds for the purpose of resale or use in production. For a manufacturer, inventory includes raw materials, components, semi-finished goods and completed products awaiting dispatch. For a retailer, inventory is simply the goods purchased for resale.
库存（又称存货）是指企业为转售或用于生产而持有的原材料、在制品和成品。对制造商而言，库存包括原材料、零部件、半成品和待发运的成品。对零售商来说，库存就是购入待售的商品。
2. Types of Inventory | 库存类型
Businesses hold several categories of inventory depending on their stage in the production process. The table below summarises the main types.
企业根据其在生产过程中的阶段持有若干类别的库存。下表总结了主要类型。
| Raw materials | 原材料 – 生产所需的基本投入 |
| Work-in-progress (WIP) | 在制品 – 尚未完成的半成品 |
| Finished goods | 成品 – 已完工、待销售的最终产品 |
| Consumables (MRO) | 消耗品 – 维护、修理和运营所需物资 |
Accurate classification is important because different types of inventory have different holding costs and reorder patterns.
准确分类很重要，因为不同类型的库存有不同的持有成本和再订货模式。
3. Why Hold Inventory? | 持有库存的原因
Businesses hold inventory to ensure they can meet customer demand without delay. Stock-outs can result in lost sales, penalty costs and damage to brand reputation.
企业持有库存是为了确保能够及时满足客户需求。缺货可能导致销售损失、罚款成本以及品牌声誉受损。
Holding buffer (safety) stock allows production to continue smoothly when suppliers face delays or there is a sudden surge in demand. It also enables bulk-buying discounts and reduces ordering costs.
持有安全库存使企业在供应商延迟或需求突然激增时仍能保持生产平稳。它还能获得批量采购折扣并降低订货成本。
In some industries, inventory is held to accommodate seasonal fluctuations, e.g. toy manufacturers building stock ahead of the Christmas season.
在某些行业，持有库存是为了应对季节性波动，例如玩具制造商在圣诞节前建立存货。
4. Inventory Costs | 库存成本
Inventory costs fall into three main categories: holding costs, ordering costs and stock-out costs. Holding costs include warehousing, insurance, obsolescence and the opportunity cost of tied-up capital. Ordering costs cover administration, delivery charges and inspection. Stock-out costs represent lost contribution, emergency orders and reputational harm.
库存成本主要分为三类：持有成本、订货成本和缺货成本。持有成本包括仓储、保险、陈旧损失以及占用资金的机会成本。订货成本包括行政管理、运输费和检验费。缺货成本则包括贡献毛利损失、紧急订单和声誉损害。
A business must balance these costs. Holding too much stock raises holding costs; holding too little increases the risk of stock-out costs.
企业必须平衡这些成本。持有过多库存会增加持有成本；持有过少则会增加缺货风险。
5. Inventory Control Charts | 库存控制图
An inventory control chart is a visual tool that tracks stock levels over time. It typically shows the maximum stock level, reorder level, minimum (buffer) stock level and lead time. When stock falls to the reorder level, a new order is placed. The delivery arrives after the lead time, just before stock reaches the buffer level.
库存控制图是一种追踪存货水平随时间变化的可视化工具。它通常显示最大库存量、再订货水平、最低（安全）库存量和提前期。当库存降至再订货水平时，便下达新订单。货物在提前期结束时送达，正好在库存触及安全库存水平之前到货。
A typical chart can be interpreted as follows: if maximum stock is 800 units, reorder level is 300 units and buffer stock is 100 units, with a lead time of 2 weeks, the average weekly usage is (300 – 100) / 2 = 100 units.
典型图表可以这样解读：若最大库存为800件，再订货水平为300件，安全库存为100件，提前期为2周，则平均每周用量为 (300 – 100) / 2 = 100件。
6. Reorder Level and Buffer Inventory | 再订货水平与安全库存
The reorder level determines when a new order should be placed. It is calculated using the formula:
Reorder level = (Lead time × Average usage) + Buffer stock
再订货水平决定何时应下达新订单。其计算公式为：
再订货水平 = (提前期 × 平均用量) + 安全库存
Buffer (safety) stock acts as insurance against unexpected spikes in demand or supply delays. The larger the buffer, the lower the risk of stock-out, but the higher the holding cost.
安全库存充当应对需求意外激增或供应延迟的保险。安全库存越大，缺货风险越低，但持有成本也越高。
In a JIT system, buffer stock is minimised to virtually zero, which places enormous pressure on reliable suppliers and accurate demand forecasting.
在准时制（JIT）系统中，安全库存被降至几乎为零，这对供应商的可靠性和需求预测的准确性提出了极高要求。
7. Economic Order Quantity (EOQ) | 经济订货量
The Economic Order Quantity model helps a business decide the optimal order size that minimises the total inventory costs. The classic EOQ formula is:
经济订货量模型帮助企业确定最优订货批量，以最小化总库存成本。经典的EOQ公式为：
EOQ = √( (2 × D × O) / C )
where:
D = annual demand (units)
O = ordering cost per order
C = holding cost per unit per year
其中：
D = 年需求量（件）
O = 每次订货成本
C = 每件每年的持有成本
EOQ assumes constant demand, fixed ordering and holding costs, and no bulk discounts. Although simplifying reality, it provides a useful starting point for stock-level decisions.
EOQ假设需求恒定、订货和持有成本固定，且无批量折扣。尽管简化了现实，它仍为库存水平决策提供了有用的起点。
8. Just-in-Time (JIT) Inventory Management | 准时制库存管理
Just-in-Time is a lean-production philosophy that aims to have materials arrive exactly when they are needed in the production process, eliminating the need for large buffer stocks. JIT relies on excellent supplier relationships, flexible workforce and zero-defect quality management.
准时制是一种精益生产理念，旨在让物料在生产需要的那一刻恰好抵达，从而消除对大量安全库存的需求。JIT依赖于良好的供应商关系、灵活的员工队伍和零缺陷质量管理。
The advantages of JIT include reduced holding costs, less space required for warehousing, lower risk of obsolescence and improved cash flow. However, it carries significant risks: any disruption in supply or a sudden increase in demand can halt production, damaging customer relations.
JIT的优点包括降低持有成本、减少仓储空间需求、降低陈旧风险以及改善现金流。但它也带来显著风险：任何供应中断或需求突然增加都可能导致生产停滞，损害客户关系。
9. Inventory Turnover Ratio | 库存周转率
The inventory turnover ratio measures how many times a business sells and replaces its stock over a given period. It is calculated as:
库存周转率衡量企业在一定时期内销售和更换存货的次数。其计算公式为：
Inventory Turnover = Cost of Goods Sold / Average Inventory
A high turnover indicates efficient stock management and strong sales, while a low turnover may signal overstocking or poor sales performance. Managers compare this ratio against industry benchmarks to judge operational efficiency.
高周转率表明库存管理高效且销售强劲，低周转率则可能意味着存货积压或销售表现不佳。管理者会将这一比率与行业基准对比，以判断运营效率。
10. Managing Inventory for Cash Flow | 库存管理对现金流的影响
Inventory ties up working capital. Excessive stock means cash is locked in goods that are not generating revenue, which can strain liquidity. Conversely, holding too little stock to save cash may lead to stock-outs, lost sales and ultimately lower cash inflows.
库存会占用营运资金。过多的库存意味着现金被束缚在尚未产生收入的货物中，从而可能加剧流动性压力。相反，为节省现金而持有过少库存则可能导致缺货、销售损失并最终减少现金流入。
An effective inventory management system constantly monitors reorder levels, demand forecasts and cash flow projections, ensuring the right balance between availability and cost.
有效的库存管理系统持续监控再订货水平、需求预测和现金流预测，确保在可得性与成本之间取得恰当平衡。
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