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Beyond the Usability Lab by Donna Tedesco, Thomas Tullis, William Albert

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6.1.3. Efficiency

So far we've looked at task success and task time separately. But in many cases what you care about most is the two of them together. That's what efficiency measures are all about: a way of combining task success and task times into a single measure that represents task success per unit of time. A participant who gets five tasks correct in 5 minutes is being more efficient than one who gets five tasks correct in 10 minutes.

Number of tasks correct per minute

One simple measure of efficiency, which we find most managers, developers, and other nonusability geeks can relate to easily, is the number of correct tasks per minute. You can calculate this for each participant:

  • Number of correct tasks/total time

Total time is ...

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