The principles of Usability

Defining usability is a difficult and sometimes contentious task. Many people understand something slightly different from the term, and I don't feel like tackling that issue just yet...

As a non-functional requirement, the measurement of usability is not direct and therefore not easy (which explains why there are so many quacks around making it up as they go along). However, it is generally accepted (and ratified by ISO 9241-11) that in order to measure usability you must measure the following factors of a system:

• Effectiveness
• Efficiency
• User satisfaction

Effectiveness means that your users should be able to accomplish the task they have in mind; your system should not fail and should not prevent users from doing their jobs.

Efficiency means that your users should be able to accomplish their tasks without jumping through all sorts of unneccesary hoops, but you knew that anyway.

User satisfaction is the hardest to define and measure but the easist to understand. A totally subjective measure relating to the other two above, it simply means that people should finish using your system without wanting to smash things.

There are plenty more factors and measures of usability, but most of them can be traced back to these 3; which is why they are in the ISO definition. Examples of important ones are:

• Learnability (people easily learn how to use your system)
• Memorability (people remember what they learned)
• Generalisability (people can apply techniques learned in other situations so don't need to learn to use your system)

There are so many more I suppose I'll carry on doing entries for chunks of them as I go along...