12.1 Definition

A definition of usability from ISO 9241-11:

[...] the extent to which a product can be used by specified users to achieve specific goals with effectiveness, efficiency and satisfaction in a specified context of use.

12.2 Designing a user study

12.2.1 Dependent vs. independent variables

Dependent variable: 
what you measure

Independent variable: 
what you manipulate (what you compare)

12.2.2 Types of data

12.2.3 Within-subjects vs. between-subjects

within-subject study:

between-subject study

12.2.4 Counterbalancing

Problem: Order of task performance can impact results

Solution: counterbalance