Little’s Law lets a Lean Six Sigma team check the arithmetic behind a process-improvement proposal: average work in progress equals average throughput multiplied by average time in the process. MIT’s Lean Thinking Part II, published in 2012 on ocw.mit.edu, connects inventory to process flow through this relationship.
För utövare som följer Lean Six Sigma Internationals Certifieringsprogram, the calculation informs a specific decision: how much inventory the process needs to sustain its required output. An inventory-reduction proposal needs to distinguish work undergoing processing from work waiting for the next operation.
Little’s Law: three averages within one boundary
Average WIP = average throughput rate × average flow time. Rearranging gives average flow time = average WIP ÷ average throughput rate. John D. C. Little’s 2011 review in Operations Research, available at people.cs.umass.edu, explains both formulations and their use in operations management.
For this Lean Six Sigma analysis, “flow time” means elapsed time between the agreed entry and exit points, including waiting. The project’s measurement plan needs that definition alongside its definition of ledtid. MIT’s 2012 material on ocw.mit.edu stresses the need to establish local time definitions because terminology varies.
Step 1: Establish the SIPOC boundary
The SIPOC discussion establishes the unit being counted. In the worked example below, that unit is one pretzel. Measurement starts at entry into cooling and ends when cooling finishes. Oven production and packaging sit outside this calculation.
Entry: cooling begins
WIP: all units inside cooling
Flow time runs from entry to exit
Exit: cooling finishes
A queue after cooling needs its own location on the Value Stream Mapping. Separating it from cooling residence time gives the sponsor a clear choice of project scope: investigate the cooling requirement or the downstream delay.
Step 2: Make the measurement window defensible
Little’s 2011 review at people.cs.umass.edu distinguishes time-average WIP from the average time experienced by individual units. It also addresses unfinished work at observation-window boundaries. Dividing a single inventory snapshot by a short-period completion rate leaves those measurement issues unresolved.
For a Lean Six Sigma baseline, agree these measurement rules with the Process Owner:
- Calculate time-weighted WIP from inventory changes and their durations, or use a documented sampling method.
- Measure departures over the same operating window, with consistent time units.
- Reconcile opening inventory, arrivals, departures and closing inventory.
- Identify unfinished units before interpreting completed-unit flow times.
Accumulating inventory belongs in the DMAIC baseline review as an unresolved imbalance. A steady-operation forecast must wait until the team has explained that imbalance and assessed the effects of unfinished work at the observation window’s boundaries.
Step 3: Calculate cooling WIP from a published example
The University of Zurich’s 2024 process-analysis material on business.uzh.ch uses a bakery producing 10,000 pretzels per hour, with six minutes required for cooling. The teaching example makes no claim of measured Lean Six Sigma project savings.
First, convert six minutes to hours: 6 ÷ 60 = 0.1 hour. Little’s Law gives 10,000 pretzels/hour × 0.1 hour = 1,000 pretzels in cooling, the answer reported in the university material on business.uzh.ch.
The Lean Six Sigma worksheet can also check the arithmetic in reverse: 1,000 pretzels ÷ 10,000 pretzels/hour = 0.1 hour, or six minutes. Reusing the published inputs from business.uzh.ch confirms consistency; it supplies no independent validation of cooling performance.
Step 4: Separate necessary residence time from avoidable waiting
Under the pretzel example’s assumptions, maintaining output and cooling duration requires the calculated inventory. A lower inventory target requires a change in throughput or residence time. The constraint follows from the Little’s Law formulation in the review at people.cs.umass.edu.
Before setting a WIP-reduction target, take the Gemba-vandring to the cooling exit. Establish whether pretzels leave when cooling finishes or remain there awaiting packaging. Timestamps separating those events give the orsaksanalys evidence of any downstream delay.
Shortening cooling requires a separate pilot test, with a product-quality acceptance measure alongside throughput. A trial that changes only release timing must retain the cooling requirement.
Step 5: Build the release decision into the control plan
Little’s Law describes averages. The calculated average inventory does not establish the maximum space required when operating conditions vary, a distinction the University of Zurich material on business.uzh.ch makes in its discussion of stochastic processes.
The styrplan needs to track WIP against throughput, with a quality balanserande mått to detect damage from process changes. It also assigns responsibility for investigating an inventory rise and specifies the evidence needed to change release rules. Keeping the original SIPOC boundary visible during review prevents a team from claiming success by moving a queue outside the measured area.
Lean Six Sigma Internationals Certifieringsprogram cover process-flow analysis and DMAIC problem solving through a progressive pathway under its LSSx.0 standard, with program details at leansixsigma.org. The appropriate level depends on the analytical decisions assigned to the practitioner, including evaluating inventory targets and defining pilot acceptance measures.
