As designers, we assume users will behave the way we expect them to behave. Unfortunately for us, humans are unpredictable … attempts to model behavior typically predict less than 10% of actions. Consider the Theory of Planned Behavior, which suggests behaviors depend on an individual’s attitude, subjective norm (how do others feel about the behavior), and perceived behavioral control (do I believe I can successfully perform the behavior). All of these factors may differ from person to person, be influenced by their context and the objects with which they are working, and typically can only account for a small portion of the variability.
Therefore, we need to see behavior. Putting a product in front of a human and giving them a chance to use it – to behave – yields insights that cannot be predicted or hypothesized without testing. Lesson: User testing and iteration matters.