GOLD STANDARD · QUEUEING THEORY · HOW X WORKS
Keep a recurrence library useful when the evidence no longer fits yesterday’s class boundaries
A recurrence-class library is a living model of queue mechanisms, not a permanent filing cabinet. As architectures, arrival mixes and measurements change, a once-useful class can become too broad, too narrow or internally contradictory. Class-library drift is the growing mismatch between a documented boundary and the cases now assigned to it. A disciplined boundary split repairs that mismatch while retrospective reclassification preserves the history of every earlier decision.
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Recognise class drift
Test and split boundaries
Reclassify without erasing history
Transfer and qualify
Use FAQs, sources and connected routes
CHAPTER 1 OF 12 · RECOGNISE DRIFT
1. A class can drift even when its name never changes
A class boundary is a claim about which events share a mechanism, operating regime and decision consequence. Drift appears when new cases accumulate that fit the label but not the claim. The symptom may still look familiar while the causal pathway, controllability or required evidence has changed.
Separate population drift from documentation drift. Population drift means the event-generating system changed. Documentation drift means reviewers gradually used the class more broadly than its written boundary. Both need repair, but only the first necessarily implies a changed queueing process.
CHAPTER 2 OF 12 · RECOGNISE DRIFT
2. Version the class contract and its exclusions
Every class should state what it includes, what it excludes, the measurements that discriminate it, the control that responds to it and the date from which the definition applies. A reviewer should be able to reconstruct which definition was available when an event was classified.
| Contract field | Question | Drift signal |
|---|---|---|
| Mechanism | What generates excess work or delay? | Two causal paths now share one label |
| Regime | When can the mechanism operate? | Assignments appear outside the declared regime |
| Signature | Which measurements should agree? | Residual pattern becomes bimodal |
| Consequence | What decision follows? | One class now triggers incompatible actions |
| Exclusion | Which look-alike does not belong? | A former exclusion is routinely admitted |
| Version | Which boundary governed the decision? | Historical records cite no definition date |
Keep definitions compact enough to use during an incident. The long evidence record can remain linked, but the boundary test itself should be visible and operational.
CHAPTER 3 OF 12 · TEST THE BOUNDARY
3. Look for structured disagreement, not isolated mistakes
Audit a sample of recent and historical assignments without showing the original label. Ask reviewers to classify each event from evidence alone and state which feature decided the result. Repeated disagreement around the same cases is more informative than one reviewer’s occasional error.
Plot within-class residuals by time, regime and mechanism signature. A stable class may have wide magnitudes yet one coherent residual pattern. A split candidate often shows two persistent clusters or a change in which features predict recovery.
CHAPTER 4 OF 12 · TEST THE BOUNDARY
4. Choose among clarification, split, merge and new-class admission
Clarify when the mechanism is unchanged but wording is ambiguous. Split when one class contains distinct mechanisms or decisions. Merge when two labels no longer support different diagnoses or actions. Admit a new class when the candidate does not inherit either existing evidence model.
Do not split merely to improve apparent accuracy. A useful split changes a diagnosis, control, evidence requirement or reopening rule. If both children would always be managed identically, a tagged subtype may be enough.
CHAPTER 5 OF 12 · WORK THE SPLIT
5. Work a class that hides two overload mechanisms
A service team reviews twelve events filed as “launch overload”. Six show a sharp external arrival burst, high server occupancy and rapid recovery after added capacity. Six show moderate external arrivals, rising retries and slow recovery until a client timeout loop is disabled.
| Check | Events 1–6 | Events 7–12 | Boundary effect |
|---|---|---|---|
| External arrivals | Large one-off burst | Moderate | Separates demand source |
| Retry share | Stable below 5% | Rises above 35% | Separates feedback mechanism |
| Server occupancy | Near saturation | Intermittent | Weakens one-class model |
| Effective control | Temporary capacity | Retry suppression | Different action |
| Blind reviewer agreement | 5 of 6 burst class | 6 of 6 retry class | Supports stable split |
| Historical label | Launch overload | Launch overload | Requires non-destructive reclassification |
The evidence supports two children: exogenous launch demand and retry-amplified feedback. The parent label can remain as a family route, but it should no longer be the final diagnostic class.
CHAPTER 6 OF 12 · MAKE THE CHANGE
6. Publish a boundary-change record before relabelling cases
Record the old definition, new definitions, reason, effective date, owner, review sample and decision consequences. State whether historical metrics will be recomputed and whether old reports remain valid under their original definitions.
Freeze the source evidence used for the change. A later reviewer should be able to distinguish evidence that justified the split from evidence collected after the new classes existed.
CHAPTER 7 OF 12 · MAKE THE CHANGE
7. Reclassify history with dual labels and provenance
Retain the original label, original decision date and original rule version. Add the new label, reclassification date, reviewer, evidence window and confidence. This dual record prevents a contemporary definition from being falsely presented as knowledge available in the past.
If evidence is insufficient, mark the historical item unresolved under the new scheme rather than forcing it into a child. Report counts both as originally decided and as retrospectively mapped whenever the distinction matters.
CHAPTER 8 OF 12 · TRANSFER THE METHOD
8. Changed-context transfer: clinical flow and cloud services
A hospital may split a boarding-delay class into bed-unavailability and discharge-processing mechanisms. A cloud service may split latency incidents into external demand bursts and retry feedback. Both can use versioned boundaries, blind reassignment and dual labels.
The features, consequences and review authority differ. Transfer the governance method, not the same thresholds, labels or acceptable uncertainty.
CHAPTER 9 OF 12 · QUALIFY THE LIBRARY
9. Validate the revised classes on held-out cases
Use cases that were not part of the split decision. Measure reviewer agreement, unresolved rate, false merges, needless splits and whether the chosen class leads to the correct control. Check performance across regimes rather than only in the dominant operating period.
Schedule an early review after the split. A boundary that works on the discovery sample can fail when new regimes appear. Reopen it when residual clusters, unresolved cases or control mismatches exceed the declared tolerance.
CHAPTER 10 OF 12 · QUALIFY THE LIBRARY
10. Limits and common misconceptions
A cluster is not automatically a mechanism. A clean retrospective split can reflect richer modern measurements that were absent historically. Reclassification does not prove the original decision was negligent, and a changed label does not change the event that occurred.
Do not delete inconvenient parent cases, tune boundaries until every example is easy, or use today’s evidence as if it existed earlier. A library can remain useful with explicit unresolved cases and provisional boundaries.
CHAPTER 11 OF 12 · CHECK AND CONTINUE
11. Questions readers often ask
Should every old report be rewritten after a split?
No. Preserve original reports and add a mapping note where the new distinction affects interpretation. Recompute only the summaries whose decision meaning genuinely depends on the revised boundary.
When is a subtype better than a separate class?
Use a subtype when the shared mechanism and control remain primary and the distinction mainly supports reporting. Use separate classes when diagnosis, evidence or action diverges materially.
CHAPTER 12 OF 12 · CHECK AND CONTINUE
12. Evidence, connected routes and the next check
NIST process-monitoring guidance frames control as comparison with historical performance. NIST remedial-action guidance separates chance signals, recording errors, bias changes, variability changes and process reevaluation. NIST run-order residual guidance shows how temporal structure can reveal drift that a pooled fit hides.
Continue through recurrence-class libraries and new-item escalation, retired-rule archives, and residual-uncertainty registers. The next check is a blind reassignment of held-out cases under both the old and proposed boundaries.