WIP and lead-time reduction

Learning objectives

• Apply Little's Law to link WIP, throughput and lead time.
• Diagnose queue, batch and information delays that inflate WIP.
• Cut WIP without starving downstream stations.

Problem and CTQ

Excess WIP hides defects, consumes cash and space, and extends lead time. Little's Law: WIP = throughput × lead time (with a defined boundary and status).

DMAIC path

• Define system boundary and status (in-line, in-queue, in-repair).
• Count WIP with timestamps and map queues, batch/release rules, imbalance, rework and information delays.
• Quantify Process Cycle Efficiency (value-add time / total lead time).

Improvement choices

• Smaller transfer batches, pull/WIP caps, pacemaker scheduling.
• Flow cells, constraint management, FIFO lanes.
• Rapid abnormal response so pulls are honoured.

Control plan

• WIP units and WIP days, throughput, lead time, PCE, aging, starvation/blocking, FPY and OTD.

Garment-factory example

A sewing module holds 2,300 WIP against 900 units/day throughput = 2.55 days lead time. A WIP cap of 1,200 and 20-piece bundles cut lead time to 1.33 days with FPY unchanged.

Common mistakes

• Cutting buffers abruptly without process stability.
• Shifting inventory outside the measured boundary and calling it a WIP reduction.