Practical issues in usability measurement

Partial goal achievement. In some cases goals may be only partially achieved, producing useful but suboptimal results (for example, for a travel Web site, not finding the cheapest air ticket). In a formative test, partial success is often judged by counting the number of assists to obtain a correct result. But for summative testing, the task is complete when the user declares that they are finished. Suboptimal results can then be scored in terms of requisite components (e.g., up to 50 percent for the correct date and time and up to 50 percent for the best fare). Overall effectiveness would be the mean value across participants.