Effectiveness

Effectiveness is one of the three pillars of usability defined in the ISO 9241 standard, alongside efficiency and satisfaction. It measures whether users can actually complete their goals, completely and accurately, when using a product.
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It is typically captured with task-success metrics: completion rate, error rate and the accuracy or quality of the outcome. A task a user cannot finish, or finishes with errors, signals low effectiveness regardless of how fast or pleasant the attempt was.

Why does it matter?

Effectiveness answers the most fundamental question about a product: does it let people do what they came to do? A beautiful, fast interface that fails this test still fails its users.
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Separating effectiveness from efficiency and satisfaction lets teams diagnose problems precisely — for example, a flow that is enjoyable and quick but frequently ends in failure.

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