HCD.10:10 - Rationale

The useful unit is not difficulty but a relation between a target action, a changed condition, and an observation. Variation, timing, retrieval, sources, support, task mixing, and challenge can each alter that relation in different ways. Keeping them separate makes a progression revisable and lets appropriate tools remain part of human performance.

A later practice observation is still practice evidence. Its date, unfamiliarity, or reduced support does not alone establish transfer or retention. Explicit history and boundaries let HCD.11–HCD.13 make the stronger inference without reconstructing hidden schedule changes.