V-AC Axis Split
Pattern guidance — when a verification-AC group mixes deterministic axis (rule match) and statistical axis (rate threshold over a window), split upfront into two groups.
Overview
A verification-AC group must not commingle checks that pass by invariant rule with checks that pass by observed rate. The two axes answer different questions; mixing them yields false confidence at the quality-review step.
The Pattern
When a single group contains both deterministic entries (for example, «schema matches XSD», «HTTP status is 200») and statistical entries (for example, «p99 latency below 500 ms», «error rate below 1 % over one hour»), split into two separate groups before finalising the wishlist.
Why Split
Deterministic and statistical validity rely on different evidence chains. A rate-based criterion grouped with rule-based checks masks that the deterministic axis always passes while the statistical axis is the real uncertainty source.
How to Apply
- Tag each entry as deterministic (unambiguous rule) or statistical (threshold over a window).
- If both axes coexist, split into a deterministic group and a statistical group.
- A statistical entry must cite the measurement window, sample size, and confidence interval.
- A deterministic entry must be tied to a spec-runner assertion or an equivalent monotonic check.