6.2 Process Analysis Tools and Value Stream Mapping

In the Measure phase of Six Sigma DMAIC (Define-Measure-Analyze-Improve-Control), process flow analysis and Value Stream Mapping (VSM) serve as foundational techniques for establishing operational baselines. Before a Black Belt can quantify variation or process capability, the team must thoroughly map how work, materials, and information physically flow through the system. Value Stream Mapping expands traditional process flowcharting by capturing systemic relationships, inventory build-ups, information signals, and detailed time metrics across the end-to-end value chain.

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Value Stream Mapping Architecture & Icons

A Value Stream encompasses all actions—both value-added and non-value-added—required to transform raw materials or initial customer requests into finished goods or completed services. Unlike standard high-level SIPOC (Suppliers, Inputs, Process, Outputs, Customers) diagrams or deployment (swimlane) flowcharts, a Value Stream Map explicitly couples material flow (moving from left to right across the lower half) with information flow (moving from right to left across the upper half).

Standard VSM Symbol Categories

• Process Boxes: Represent individual operational work centers where transformation or handling occurs. Each box contains a dedicated Data Box recording empirical process parameters.
• Data Boxes: Contain quantitative metrics collected during Gemba walks: Cycle Time ($C/T$), Changeover Time ($C/O$), Uptime (Machine Availability %), Work-in-Process (WIP) inventory, batch sizes, number of operators, and available shift time.
• Inventory Triangles: Represent material queues, storage buffers, or staging areas between process steps, labeled with the exact quantity of inventory ($I$).
• Information Flow Lines: Depict manual information flows (straight thin arrows) or electronic data transfers (zigzag lightning arrows) such as ERP/MRP scheduling, production control orders, and Kanban signals.
• Timeline Ladder: Positioned at the bottom of the map, contrasting Value-Added Processing Time (on the lower step) against Non-Value-Added Inventory Lead Time (on the upper step).

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Process Time Definitions & Mathematical Formulations

Precise statistical definition of time metrics is critical for Lean Six Sigma Practitioners to quantify waste and establish production cadence:

1. Takt Time

Derived from the German word Taktzeit (meter or beat), Takt Time is the maximum allowable time per unit of product or service required to meet customer demand. It is not the time it takes to build a product, but rather the required pace of production.

$\text{Takt Time} = \frac{\text{Net Available Operating Time per Period}}{\text{Customer Demand Quantity per Period}}$

Where:

• Net Available Operating Time = Total gross shift time minus planned downtime (rest breaks, meal times, shift handover, planned maintenance, and scheduled team meetings).
• Customer Demand Quantity = Total units required by the customer over the identical operating period.

Example Calculation: A manufacturing cell operates on a single 8-hour (480-minute) shift per day. Planned downtime includes two 15-minute rest breaks and a 30-minute lunch break. Daily customer demand is 420 units.

• Net Available Time = $480\text{ min} - (15 + 15 + 30)\text{ min} = 420\text{ minutes} = 25,200\text{ seconds}$.
• Takt Time = $\frac{25,200\text{ seconds}}{420\text{ units}} = 60.0\text{ seconds/unit}$. The production cell must produce exactly one conforming unit every 60 seconds to satisfy customer demand without creating overproduction waste.

2. Cycle Time ($C/T$)

Cycle Time is the actual measured time elapsed between the completion of consecutive units at a specific process step. It measures the operational speed of a process step.

• Value-Added Cycle Time ($VA C/T$): The portion of cycle time dedicated to physically transforming the part.
• Machine Cycle Time: The time required for automated equipment to complete its processing cycle.
• Operator Cycle Time: The hands-on work time required by an operator to load, unload, inspect, and run a process step.

3. Lead Time ($L/T$) & Little's Law

Process Lead Time (or Throughput Time) is the total calendar or clock time elapsed from the moment an order or part enters the process stream until it is fully completed and delivered. Inventory stored between process steps directly inflates Lead Time. Utilizing Little's Law, inventory quantities are converted into equivalent lead time:

$\text{Inventory Lead Time} = \frac{\text{Work-in-Process (WIP) Inventory Quantity}}{\text{Daily Customer Demand (or Daily Output Rate)}}$

$\text{Total Process Lead Time} = \sum \text{Inventory Lead Times} + \sum \text{Process Cycle Times}$

4. Process Cycle Efficiency (PCE)

Process Cycle Efficiency measures the proportion of total lead time that is genuinely spent adding value for the customer:

$\text{PCE} = \left( \frac{\text{Total Value-Added Time}}{\text{Total Process Lead Time}} \right) \times 100\%$

In traditional non-lean processes, PCE is frequently below $5%$, meaning that over $95%$ of process lead time consists of waiting, storage, and queue delays. Lean world-class benchmark operations achieve a PCE greater than $25%$.

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Categorization of Process Activities

Six Sigma practice categorizes every process task into one of three distinct operational buckets to eliminate waste and optimize flow:

The 8 Wastes of Lean (TIMWOODS)

1. Transportation: Excessive movement of materials, parts, or documents between locations.
2. Inventory: Excess raw materials, WIP, or finished goods exceeding immediate operational requirements.
3. Motion: Unnecessary physical movement of personnel (searching, bending, stretching, walking).
4. Waiting: Idle time spent waiting for materials, approvals, machine cycles, or upstream work.
5. Overproduction: Producing items faster, earlier, or in greater quantities than required by customer Takt time (the worst form of waste).
6. Overprocessing: Performing redundant, overly precise, or unnecessary process steps beyond customer specification.
7. Defects: Rework, scrap, inspection overhead, and corrective efforts caused by non-conforming work.
8. Skills / Talent Underutilization: Failing to engage employees' problem-solving capabilities, intelligence, and creative ideas.

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Selecting Among the Process Analysis Tools

Body of Knowledge topic V.A.2 asks Black Belts to select, use, and evaluate process analysis tools at the Evaluate level, naming value stream maps, process maps, work instructions, flowcharts, spaghetti diagrams, and the gemba walk. Value stream mapping is treated above; the point of this topic is choosing the right instrument for the question being asked.

Spaghetti diagrams

A spaghetti diagram is drawn on a scale layout of the work area. Follow one operator, one part, or one document through a full cycle and draw a continuous line along the actual path taken. The result makes motion and transportation waste immediately visible, and it quantifies them: measure the total path length, multiply by the number of cycles per shift, and the walking becomes minutes of paid time.

A worked figure of the kind these produce: an operator whose path measures 132 metres per cycle, running 34 cycles per shift, walks 4.5 km per shift. At a normal walking pace that is roughly 55 minutes of paid time per shift spent moving rather than working -- a business case that a process map alone would never surface.

Spaghetti diagrams are the natural precursor to a cell redesign, a 5S event, or a point-of-use material relocation, and re-drawing the diagram after the change is the simplest possible before-and-after proof.

Comparing the documented process with the real one

The most valuable analysis in this topic is the comparison between the work instruction and the gemba observation. Three outcomes, each with a different action:

Skipping this comparison is how teams end up modelling and improving a process that exists only on paper. Walk the process before mapping it, and map what you saw rather than what you were told.

Conducting a gemba walk

Go to the actual place, look at the actual process, and talk to the people doing the work. Practical discipline: observe a full cycle without interrupting, then ask questions; ask what happens when something goes wrong rather than whether problems occur; follow one unit end to end rather than watching one station; and observe on more than one shift, because shift-to-shift differences are frequently the largest source of variation in the whole study.