Whitney Quesenbery用5个E开头的单词来描述可用性的5个维度，分别是：Effective，Efficient，Engaging，Error Tolerant，Easy to Learn。
Effective 指的是效力。如果一个系统或者设备无法让用户完成他想要做的工作，无论任务难易或者所花费时间的长短，那么这个系统的效力是很低的。如果我们想测量系统的效力，需要先定义什么是成功的完成一个任务，不管是直接或者曲折的。
Efficient 指的是效率，这个很容易理解。完成任务的时间是衡量效率的主要指标。
Egaging 指的是满意程度，包含了这个界面在多大程度上吸引用户去使用，以及使用中的愉悦感觉，当然还有使用后的满意程度。
Error Tolerant 指的是系统防止错误以及从错误中恢复的能力。
Easy to learn 指的是可学性。某些系统的使用的频率很低，用户可能隔很久才会再用一次。怎么保证用户事隔很久之后还能很容易的再次使用这个系统呢？可学性是一个重要的指标。
Effective
The completeness and accuracy with which users achieve their goals.
Effective is the first E. If a user cannot actually do the thing he or she set out to do, it
probably doesn’t matter whether the experience was short or long, easy or hard. In the end,
they have failed to complete their tasks or meet their goals. If we want to be able to
measure effectiveness, we have to know how people define success or usefulness, whether this is
relatively straight forward or more subtle.
Efficient
The speed (with accuracy) with which this work can be done.
Efficiency may be something that is carefully defined, for example in a call center where
operators are measured on the number of calls they can handle in a day. It may be a
subjective judgement of when a task is taking “too long” or “too many clicks.”
Engaging
How pleasant, satisfying or interesting an interface is to use
“Engaging” replaces “satisfaction,” looking for a word that suggests the ways that the
interface can draw someone into a site or a task. It also looks at the quality of the
interaction, or how well the user can connect with the way the product is presented and
organized.
Error Tolerant
How well the product prevents errors, and helps the user recover from any that do
occur
It would be lovely to say “error free” or “prevents errors” but mistakes and accidents and
misunderstandings will happen. The cat nudges the mouse as you click. You misread a link
and need to find your way back, or enter a number with a typo. The real test is how helpful
the interface is when an error does occur.
Easy to Learn
How well the product supports both initial orientation and deeper learning
A product may be used just once, once in a while, or on a daily basis. It may support a task
that is easy or complex; and the user may be an expert or a novice in this task. But every
time it is used the interface must be remembered or relearned, and new areas of the
product may be explored over time.
