A production method in which large numbers of items (batches) are processed as a whole and then, regardless of whether they are needed immediately, are passed on to the next process, where they wait in a queue until it is that process’s turn. Batch-and-queue is the dominant model in mass production and the direct opposite of continuous flow.

Why batch-and-queue arises
The logic behind batch-and-queue is understandable: by producing large quantities all at once, changeovers are minimized and machines and employees appear to be utilized optimally. At the level of an individual department or machine, batch-and-queue feels efficient. At the level of the entire system—the complete flow from raw material to customer—it rarely is.

Every batch waiting for the next process represents tied-up capital, hidden quality issues, and wasted time. A defect that arises in batch 1 is not detected until batch 1 has reached the next step; by that time, it may have already spread to hundreds of units.

The Consequences
Batch-and-queue systems structurally lead to a series of inefficiencies: long lead times because items spend most of their time waiting rather than being processed, high levels of work-in-progress between process steps, hidden quality issues that are discovered too late, and capacity that is difficult to plan because demand fluctuations are amplified upstream.

In Lean, lead time is the most important measure of system health. Research on production processes consistently shows that in a batch-and-queue environment, items are actually processed for only a small percentage of the total lead time. The rest is waiting time.

TheLean
Lean aims for continuous flow: moving one item at a time through the process, synchronized with the takt time, so that queues are eliminated and problems become immediately visible. Where continuous flow cannot be fully achieved, pull systems and supermarkets are used to limit the amount of work in progress and shorten lead time. Tools such as SMED make it possible to reduce changeover times so significantly that the economic justification for large batches disappears.

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