Inventory Policy

Inventory policy determines the volume and positioning of stock throughout a supply chain network. The discipline balances the competing objectives of maximising customer service levels and minimising the capital tied up in holding inventory. A coherent policy relies on mathematical parameters rather than subjective judgment.

ABC Classification and Stratification

The foundation of formal inventory management is the segmentation of the product portfolio. ABC classification applies the Pareto principle, categorising items based on their annual consumption value. 'A' items represent the highest consumption value, often accounting for 70 to 80 percent of annual spend while constituting a minor percentage of total part numbers. 'C' items account for the lowest financial consumption.

Stratification dictates the control mechanisms applied to each segment. 'A' items require tight cycle counting schedules, rigorous supplier monitoring, and frequent order cycles to minimise holding costs. Conversely, 'C' items are managed with looser controls and larger batch quantities, as the administrative cost of managing them frequently outweighs their intrinsic material value.

Economic Order Quantity (EOQ)

The Economic Order Quantity (EOQ) model determines the optimal volume to purchase or manufacture by balancing fixed ordering costs against variable holding costs. Ordering costs include the administrative burden of raising purchase orders, receiving goods, and processing invoices. Holding costs encompass warehousing space, insurance, capital depreciation, and the opportunity cost of tied-up funds.

The EOQ calculation yields the batch size that minimises total total cost. While the standard formula assumes constant demand and instantaneous replenishment, the output remains a valuable starting point for negotiations. In practice, calculated EOQ values are frequently adjusted to align with standard pallet sizes, minimum supplier order quantities, or transport container dimensions.

Safety Stock Modelling

Safety stock operates as a statistical buffer against supply chain variability. It protects operations from two distinct sources of volatility: unexpected spikes in customer demand and variability in supplier lead times. Establishing a flat percentage of expected demand as safety stock is a common anti-pattern that overprotects reliable items while underprotecting highly variable product lines.

Rigorous safety stock calculations deploy normal distribution statistics. The formula incorporates the desired service level (frequently expressed via a Z-score, where 1.645 represents a 95 percent service level), the standard deviation of historical demand, and the standard deviation of lead time. By explicitly quantifying variance, organisations isolate precisely how much inventory is required to maintain operations under stressed conditions.

Review Periods: Continuous versus Periodic

Inventory replenishment policies generally fall into two structural categories: continuous review and periodic review. Under a continuous review system, inventory levels are monitored perpetually, often facilitated by modern ERP systems. An order is triggered the moment the inventory drops below a predetermined reorder point. This method generally results in lower safety stock requirements as the system reacts instantly to consumption.

A periodic review system inspects inventory at fixed intervals—for example, every Friday afternoon. An order is placed to bring the stock back up to a target maximum level. While this approach necessitates higher safety stock to cover the review interval duration, it facilitates consolidated ordering, reducing transport costs and simplifying supplier administration.

Evaluating Policy Performance

The effectiveness of an inventory policy is measured via inventory turnover ratio and fill rate. Turnover indicates the frequency with which the complete inventory stock is sold and replaced over a year. High turnover signals efficient capital utilisation, provided the fill rate—the percentage of customer orders met from immediate stock availability—does not degrade.

When turnover drops while fill rates remain static, the policy requires recalibration. This scenario indicates that safety stock parameters have been set unnecessarily high, or that obsolete stock is inflating the denominator of the turnover calculation without contributing to operational performance.