Lean Concepts

Lean Concepts (CSSGB BoK I.B.1 — Apply)

Quick Answer: Lean maximizes customer value by removing waste and creating smooth flow. Core ideas include the value chain, takt time, just-in-time (JIT), Theory of Constraints (TOC), Gemba, spaghetti diagrams, and continuous pursuit of perfection. Green Belts apply these tools inside DMAIC alongside Six Sigma variation reduction.

Lean defines success as delivering what the customer values, when they need it, with the least non-value-added activity. On the CSSGB exam, I.B.1 is at the Apply level: calculate takt time, recognize flow versus push, and choose the right lean concept for a scenario.

Value, Value Chain, and Flow

Value is what the customer is willing to pay for. Activities that transform product or information toward that outcome are value-added (VA). Everything else is non-value-added (NVA) waste or necessary non-value-added (NNVA) work required by regulation or current technology.

The value chain is the end-to-end sequence that creates and delivers value—from order entry through production, logistics, and service. Lean improves the whole chain, not isolated departments. Local efficiency that floods the next process with WIP is not lean success.

Flow means work moves steadily with minimal queues, batching, and stoppages. Ideal flow is one-piece (or small-lot) movement matched to demand. Obstacles include large batches, unbalanced stations, long changeovers, and poor layout. Spaghetti diagrams and takt analysis expose those obstacles so Green Belts can redesign the path of work.

Core Lean Terms (Exam Table)

Takt Time — Definition and Worked Example

Takt time is the maximum time allowed to produce one unit to meet customer demand:

$\text{Takt time} = \frac{\text{Available production time}}{\text{Customer demand}}$

It is the customer drumbeat. Station cycle time should be at or below takt (with buffer for variation). Stations much faster create idle inventory; stations slower cannot meet demand.

Worked example

A packaging cell runs one 8-hour shift with a 30-minute paid lunch and two 10-minute breaks:

• Shift length: 8 × 60 = 480 minutes
• Non-working time: 30 + 10 + 10 = 50 minutes
• Available time = 480 − 50 = 430 minutes/day

Customer demand is 860 finished units per day.

$\text{Takt time} = \frac{430\ \text{min}}{860\ \text{units}} = 0.5\ \text{min/unit} = 30\ \text{seconds/unit}$

Interpretation:

1. Every 30 seconds, one unit must leave the process to stay on demand.
2. If the bottleneck cycle time is 42 seconds, demand cannot be met—improve the constraint (reduce cycle time, add capacity, or rebalance) before speeding non-bottleneck stations.
3. If average cycle time is 22 seconds but WIP piles up, the issue is batching, unbalanced flow, or push—apply JIT pull and flow redesign.

Use only the available-time exclusions the scenario states; do not invent efficiency factors unless given.

Just-in-Time (JIT)

Just-in-time produces and moves only what the next process needs, when it needs it, in the quantity needed. JIT replaces forecast-driven push with pull (often via kanban). Benefits include less inventory, shorter lead time, earlier defect detection, and less cash tied in WIP. Prerequisites include stable quality, reliable feeders, short changeovers, and a known takt. Green Belts often implement pull at the process level and cut lot sizes inside DMAIC Improve.

Theory of Constraints (TOC)

Theory of Constraints holds that system throughput is limited by a small number of constraints (often one primary bottleneck). The five focusing steps:

1. Identify the constraint.
2. Exploit it (maximize its effective use—no idle bottleneck).
3. Subordinate everything else to the constraint (non-constraints must not overproduce).
4. Elevate the constraint (capacity, technology, or redesign) if needed.
5. Repeat when the constraint moves.

TOC complements lean: lean removes waste broadly; TOC prioritizes where improvement most increases throughput. If a scenario shows one machine with endless queues while upstream idles, prioritize constraint management, not only general 5S.

Gemba, Spaghetti Diagrams, and Perfection

Gemba is the actual workplace. Gemba walks observe standard work, wait time, motion, and defects—rather than managing only from dashboards. Respect for people matters: observe to learn and improve, not to blame.

A spaghetti diagram traces the physical path of product, documents, or workers on a floor plan. Long, crossing, or backtracking paths indicate motion and transportation waste. After cell redesign or point-of-use storage, redraw the diagram; path length and crossings should drop.

Lean’s fifth principle is perfection: there is always more waste to remove. Green Belts lock gains with control plans and standard work, then start the next kaizen or DMAIC cycle.

How Lean Integrates with Six Sigma at Green Belt Level

Six Sigma attacks variation and defects with data and statistical tools. Lean attacks waste and flow. Together (Lean Six Sigma):

At Green Belt level you should:

• Use lean concepts to diagnose problems (Gemba, takt, constraint identification).
• Select lean countermeasures when root causes are batching, waiting, motion, or overproduction.
• Combine Six Sigma analytics when the problem is capability, measurement error, or multi-factor settings.
• Match the tool to the problem—avoid “tool worship.”

Exam tip: customer demand pace → takt; only produce when pulled → JIT/pull; slowest step limits the system → TOC; go see the process → Gemba.

A typical Green Belt sequence: walk the Gemba; draw a spaghetti diagram; compute takt (e.g., 30 s/unit); identify the sealer as the TOC constraint at 42 s; rebalance work; implement pull; then use Six Sigma tools on remaining seal defects. That is lean in action inside a Six Sigma project structure.