What Do We Mean by "Usable"? - II

Efficiency is the quickness with which the user s goal can be accomplished accurately and completely and is usually a measure of time. For example, you might set a usability testing benchmark that says "95 percent of all users will be able to load the software within 10 minutes."

Effectiveness refers to the extent to which the product behaves in the way that users expect it to and the ease with which users can use it to do what they intend. This is usually measured quantitatively with error rate. Your usability testing measure for effectiveness, like that for efficiency, should be tied to some percentage of total users. Extending the example from efficiency, the benchmark might be expressed as "95 percent of all users will be able to load the software correctly on the first attempt."

Learnability is a part of effectiveness and has to do with the user's ability to operate the system to some defined level of competence after some predetermined amount and period of training (which may be no time at all). It can also refer to the ability of infrequent users to relearn the system after periods of inactivity.

Satisfaction refers to the user's perceptions, feelings, and opinions of the product, usually captured through both written and oral questioning. Users are more likely to perform well on a product that meets their needs and provides satisfaction than one that does not Tvoicallv „c and rank products that they trv and ZT/ users are as» for problems that occur 'eveai causes and reasons product usable is never simply the abilitv  the numbere can tell us no there ,s a distinctive qualitative element to how usable somethine is a well, which is hard to capture with numbers and is difficult to pin down to do with how one interprets the data in order to know how to fix a problem because the behavioral data tells you why there is a problem. Any do tor

can measure a patient's vital signs, such as blood pressure and pulse rate 

But interpreting those numbers and recommending the appropriate course of action for a specific patient is the true value of the physician JudeinR the several possible alternative causes of a design problem, and knowing which are especially likely in a particular case, often means looking beyond individual data points in order to design effective treatment. There exist these little subtleties that evade the untrained eye.