389. CONWIP

Constant Work-In-Process: a production-control discipline that caps WIP at a constant level $𝑤$. 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.

389.1. Mechanic

1. Set WIP cap ${𝑤}^{\ast }$ (typically ${𝑤}^{\ast }>{𝑊}_0$, the critical WIP)
2. Whenever a unit finishes and exits the line, authorize a new unit to enter
3. Total WIP stays constant at ${𝑤}^{\ast }$ 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

389.2. Why CONWIP works

The PWC formula:

shows throughput approaches ${𝑟}_{𝑏}$ as $𝑤$ grows. Setting $𝑤$ generously above ${𝑊}_0$ 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.

389.3. Setting the cap

Trade-off:

• Too low $𝑤$: starvation, throughput below ${𝑟}_{𝑏}$
• Too high $𝑤$: long cycle time, lots of inventory, late deliveries

Rule of thumb: ${𝑤}^{\ast }={𝑊}_0+$ buffer. Buffer sized to cover typical variability — start with $1.5\cdot {𝑊}_0$ and adjust based on observed throughput / cycle time.

389.4. 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

389.5. Comparison with Kanban

CONWIP is simpler and adapts better to product mix; kanban gives tighter per-product control.

389.6. See also

• Kanban Sizing
• Critical WIP
• Best/Worst/PWC
• Factory Physics