Chapter 32: Time-Based vs. Event-Based Verification Strategies

Different nonconformances demand different verification approaches. The strategy you choose shapes how long your CAR stays open and what data you collect.

Time-based verification is your default for process nonconformances where recurrence is the primary risk. You select a time window—typically 30, 60, or 90 days—and monitor the metric continuously. At the end of the window, you assess whether the process remained stable and in-control.

When do you use time-based? When the root cause was a one-time event (a tool wore out, a batch of raw material was defective, a procedure was unclear) and the fix targets the process itself. You run multiple cycles of the process and verify the nonconformance doesn't reappear. A Montréal medical device manufacturer fixed a contamination issue in their clean room by replacing HVAC filters and retraining staff. They verified effectiveness by monitoring particle counts in the clean room every shift for 90 days. Time-based made sense because the risk was sporadic recontamination; continuous monitoring proved the fix worked.

Time-based verification typically uses these sample sizes:

• Low-risk nonconformances (dimension drift, minor cosmetic defects): 30-day window, daily sampling, minimum 150 units measured
• Medium-risk (escaped defects to internal customers, scrap increase): 60-day window, daily sampling, minimum 300 units measured
• High-risk (customer returns, safety nonconformances, regulatory escapes): 90-day window, daily or shift sampling, minimum 500 units measured, documented SPC chart with statistical capability analysis

Event-based verification is appropriate when the nonconformance is tied to a specific product, customer lot, or audit cycle. Instead of waiting 60 days, you verify on the next occurrence of the triggering event. This closes CARs faster when the root cause is specific and the risk of recurrence is lower after the fix.

When do you use event-based? When the nonconformance was customer-specific (a complaint on part number XYZ-100 for customer ABC), or when the issue is tied to a rare production event. A Burlington aerospace supplier had a nonconformance on a bracket heat-treat specification that only applied to one part number for one customer. They fixed the heat-treat setup and verified effectiveness on the next production run of that specific part (event-based) rather than waiting 60 days for random time-based monitoring. Event-based made sense because the issue was part-specific, not process-wide.

Event-based verification examples:

• Next production run of the affected part number: Verify 100% of output or per your sampling plan
• Next shipment to the affected customer: Inspect full lot before shipment with tightened acceptance criteria
• Next internal audit of the process: Use the audit as a verification checkpoint, documenting the CAR status and evidence in the audit report
• Next external customer audit or surveillance audit: Include CAR closure and effectiveness verification as an explicit agenda item

Key Consideration: Most Canadian manufacturers mix these approaches. You might use time-based monitoring for the first 30 days to prove the fix doesn't cause immediate reversion, then shift to event-based monitoring (next internal audit, next customer order of that part) for longer-term surveillance. This hybrid approach provides both quick closure and ongoing confidence.