Just-in-Time (JIT) is not simply a policy of carrying very little inventory. It is an operating approach built around flow, pull, short lead times, reliable quality and disciplined replenishment. Reducing inventory before improving the underlying process can expose a business to frequent shortages rather than create Lean performance.
When JIT Works Best
- Demand is reasonably visible or can be replenished from actual consumption.
- Suppliers and transport lanes are reliable.
- Lead times are short and stable.
- Quality problems are detected and corrected quickly.
- Batch sizes and setup times are low enough to support frequent replenishment.
- Inventory records and pull signals are accurate.
Core JIT Prerequisites
| Prerequisite | Why it matters | Typical control |
|---|---|---|
| Reliable suppliers | Small buffers leave less tolerance for missed deliveries | OTIF monitoring, supplier development, backup sources |
| Short lead times | Replenishment must respond quickly to consumption | Local sourcing, reduced queues, smaller lots |
| Quality at source | Defects can stop downstream flow | Incoming quality agreements, process capability, rapid containment |
| Accurate inventory | Pull signals depend on trustworthy stock data | Cycle counts, barcode/RFID controls, transaction discipline |
| Stable processes | High variability creates shortages and expediting | Standard work, preventive maintenance, capacity monitoring |
Kanban and Pull Signals
A kanban is a replenishment signal. Consumption at the downstream process creates a signal for upstream replenishment. The objective is to replace what was used rather than continuously pushing material based only on forecasts.
JIT Does Not Mean Zero Inventory
Real supply chains contain variability. Even a Lean operation may need strategically placed buffers for long lead-time items, highly variable demand, critical spares or constrained suppliers. The goal is to remove unnecessary inventory while preserving the service and resilience the process requires.
A Practical Implementation Sequence
- Map current material and information flow.
- Identify demand pattern, lead-time variation and bottlenecks.
- Improve supplier and internal process reliability.
- Reduce batch size and setup time where practical.
- Introduce pull/kanban signals.
- Reduce buffers gradually while monitoring service.
- Add resilience measures for critical or long-lead items.
KPIs to Monitor
- Supplier OTIF
- Lead-time average and variability
- Inventory days / inventory turns
- Stockout frequency
- Schedule adherence
- First-pass yield / defect rate
- Expedite cost
For the concept, benefits and risks, see Just-in-Time Inventory: How It Works, Benefits, Risks & When to Use It. For operating examples, see JIT Examples: Kanban, Supplier Delivery, Build-to-Order & Warehouse Flow.
Interview Question
Question: Why can reducing inventory too early make a JIT implementation worse?
Answer: Inventory often masks process instability, supplier unreliability and long lead times. If buffers are removed before those causes are improved, shortages and expediting increase. JIT should reduce inventory as process reliability improves.
Comments 1