Together they form usability in the broad sense; neither alone suffices
Aliases: combined usability · utility times usability · joint determination
What it is
Whether "this product works" in the broad sense is decided jointly: does the function do the right thing (utility) times does it run smoothly (usability). Either factor at zero makes the whole zero—that is the precise meaning of "neither alone suffices": not two scorecards that compensate each other, but two multiplied terms.
Why it happens
The multiplicative structure comes from the value chain: utility decides whether doing it yields output, usability decides whether the output can actually be obtained, and a break anywhere zeroes the product and voids the other link's effort. The compensation illusion exists because both links can be probed in the same user interviews: feature worshippers and interaction worshippers each hold locally consistent data while the chain breaks on the unchecked link.
Studying it
Assessments must measure both links at once: need-coverage mapping answers utility, task-based measurement answers usability, and conclusions are stated jointly ("feature X satisfies need Y and is usable by target users"). Diagnosis locates the break first: does churn come from "no such feature" or "feature too hard to use"? The interventions differ completely. TAM-tradition studies on relative weights show weights shift by context—no fixed ordering exists.
Where it stops holding
The multiplication is an approximation: features hold partial value (half a need met), usability is graded; in practice "adequate feature + decent interaction" can beat "perfect feature + broken interaction," and the reverse, depending on need rigidity and alternatives. The two are not fully independent either—feature design shapes usability (more features make simplicity harder). The claim constrains evaluation completeness, not a numeric formula.
Applying it
- Fix a two-section structure in product evaluations: need-coverage verdict and usability evidence; conclusions without both are rejected.
- In kickoff reviews, ask the breakpoint question: users can't get the outcome—is the feature missing or unusable? The answer directs the budget.
- List investment in the two links separately on roadmaps so usability work and feature gaps don't silently eat each other.