From entry effectiveness in glossary Human Computer Interaction the Basics
Effectiveness is one of the three core components (effectiveness, efficiency and satisfaction) in ISO 9241, which are widely regarded as key criteria for effective user interface design. Effectiveness is concerned with whether the things that the user wants or needs to do are possible with a system. For example in a library system, is it possible to check books in and out, or add new books. Ensuring this will typically involve user research to understand the jobs or lives of users as well as more formal requirements for a job and/or legal constraints.
Effectiveness would normally be measured in a binary fashion with a list of required functions, or use cases, that are ticked off one by one during system development and evaluation.