Value stream mapping (VSM)
A value stream map (VSM) is a visual representation of the flow of materials and information from raw material to the customer. Unlike a process flowchart (which shows steps in one process), a VSM spans the entire value stream and captures both the work time at each step and the waiting time between steps. This reveals where most of the lead time actually hides.
Key VSM metrics
| Metric | Definition |
|---|---|
| Cycle time (CT) | Time to complete one unit at a single process step |
| Lead time (LT) | Total elapsed time from start to finish through the entire value stream, including all waiting |
| Process time (PT) | Actual hands-on working time at a step (may be less than CT) |
| Inventory (units) | Work-in-process between steps; each unit of WIP = delay time |
| Value-added ratio (VAR) | Process time / Lead time × 100%; shows how much time is actually adding value |
Current-state steps (simplified):
| Step | Cycle time | Queue / wait after step |
|---|---|---|
| API dispensing | 2 h | Wait for blend room availability: 24 h |
| Blending | 4 h | Wait for compression scheduling: 48 h |
| Compression | 6 h | Wait for QC in-process approval: 24 h |
| Packaging | 3 h | Wait for QC final release: 72 h |
Total process time: 2 + 4 + 6 + 3 = 15 hours
Total wait time: 24 + 48 + 24 + 72 = 168 hours
Total lead time: 183 hours
Value-added ratio: 15 / 183 = 8.2%
Only 8.2% of the total time is actually adding value. The rest is waiting.
Standard VSM symbols
- Process box: Each manufacturing or service step with CT, changeover time, uptime, operators
- Push arrow: Material is pushed to the next step regardless of demand
- Pull arrow / supermarket: Material is pulled only when the downstream step requests it
- Kaizen burst: A starburst marking an improvement opportunity (usually on the future-state map)
- Inventory triangle: WIP between steps; shows quantity and days
- Customer/supplier box: External entities at the far right (customer) and far left (supplier)
- Timeline: Drawn below the VSM; alternates between value-added (process) time and non-value-added (wait) time
Current state vs. future state
| Current state | Future state | |
|---|---|---|
| Purpose | Document reality; calculate VAR and lead time | Design the improved process with waste removed |
| Built from | Direct observation (gemba walk) | Lean principles: pull, flow, takt time alignment |
| Shows | Where waste and delays live | The target state after improvements |
From VSM to kaizen events
After drawing the current state, identify the largest sources of wait time and inventory. Draw a kaizen burst (a starburst symbol) at each improvement opportunity on the future-state map. Each burst is a candidate for a rapid improvement event (kaizen) or a focused DMAIC project.
Common exam traps
- Lead time ≠ cycle time. Cycle time is the work time at one step. Lead time is total elapsed time through the whole stream including all waiting. Most lead time is waiting, not working.
- Value-added ratio is usually low. Single-digit percentages are not unusual; this is not a failure — it shows the improvement opportunity.
- VSM is drawn from the customer backwards. Start by understanding customer demand (takt time), then map back through the value stream to the supplier.
- VSM is not a process map. It spans multiple process steps and functions; a process map shows one step in detail.
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