Inventory Classes and Policy Selection

Why it matters

Inventory is a resource held today to meet future consumption. Two items on the same shelf do not have to be managed by the same rule. An expensive housing may have a large capital effect; the absence of a cheap fastener, on the other hand, can stop a whole assembly. Inventory classes and policy selection let you see both consequences together. You first define which loss you want to limit, then choose the review and ordering rule that fits that purpose. Classification is not a labeling contest; it is a decision tool that justifies which item gets which attention, recording frequency and replenishment assurance.

Annual consumption value is the product of the quantity consumed in a year and the unit value for the same period. This value differs from the total quantity purchased, from the amount on the shelf today and from sales revenue. ABC classification ranks items by this quantity. Class boundaries are explained according to the purpose of the business; there are no fixed percentages that arise by themselves in every data set. Giving more frequent attention to a small number of high-value items is useful. But this ranking alone does not say whether production will stop in the absence of the part, whether an alternative use is possible or how reliable replenishment is. Reading value order as risk order therefore produces a wrong result.

The regularity of demand also changes the policy choice. Of two parts with the same monthly average, one may be used every day while the other is consumed only in certain products. Watching daily movement frequently may make sense for the first part; for the second part, which shows sparse and lumpy demand, the product plan and the delivery commitment are more decisive. The coefficient of variation is the ratio of the standard deviation to the mean and helps comparison by reducing the scale difference. When the mean is close to zero, this ratio becomes unstable. Instead of explaining many zero periods with a single ratio, you need to examine separately how often demand occurs and how much arrives when it does. Although the period total looks sufficient for classification, the sequence of events is needed for policy. The same total consumption can arise as a single order at the start of the month or spread across days. In the first case the quantity that must be ready before delivery is different. Do not therefore use the annual value table in place of daily movement records. Totals set the priority; time information shows how much resource is needed at which moment and makes the chosen policy executable.

The name of the service target must also be explicit. The probability of having no shortage in a replenishment cycle and the share of total demand units filled immediately are different measures. A small number of large shortages can move these measures in different directions. The success of a policy is not only a reduction in inventory value; demand met, delay and stoppage risk are monitored within the same scope. If demand that cannot be met becomes a backorder, the debt record is kept; if the customer gives up, lost demand is recorded separately. No issue from the warehouse does not mean no demand. This distinction also forms the data basis of the later safety stock calculations.

Formula