386. CONWIP
Constant Work-In-Process: a production-control discipline that caps WIP at a constant level . New work enters only when finished work exits — a pull system but with one global cap instead of per-station kanbans.
Developed at Northwestern (Spearman, Woodruff, Hopp 1990) as a generalization of kanban.
386.1. Mechanic
- Set WIP cap (typically , the critical WIP)
- Whenever a unit finishes and exits the line, authorize a new unit to enter
- Total WIP stays constant at always
Compare:
- Push (MRP-driven): release per planned schedule, WIP fluctuates
- Kanban: each station holds local kanbans; WIP bounded but more complex
- CONWIP: one global cap; simpler, often as effective
386.2. Why CONWIP works
The PWC formula:
shows throughput approaches as grows. Setting generously above guarantees you’re close to bottleneck-limited throughput, while keeping cycle time bounded.
vs uncapped push: WIP can grow without bound under variability, blowing up cycle time.
386.3. Setting the cap
Trade-off:
- Too low : starvation, throughput below
- Too high : long cycle time, lots of inventory, late deliveries
Rule of thumb: buffer. Buffer sized to cover typical variability — start with and adjust based on observed throughput / cycle time.
386.4. Implementation
- Cards / tokens: physical or digital “authorizations” that flow with units
- Heijunka box: time-sliced authorization (with level scheduling)
- MES system: software cap enforced on releases
386.5. Comparison with Kanban
| Kanban | CONWIP | |
|---|---|---|
| WIP control | Per-station local | Global single cap |
| Mix flexibility | Limited (per-part kanbans) | Higher (any part fills the cap) |
| Implementation | More complex | Simpler |
| Per-product visibility | Better | Aggregate only |
| Best for | Repetitive, low-mix | Higher-mix, more flow-flexible |
CONWIP is simpler and adapts better to product mix; kanban gives tighter per-product control.