Based on the International Standards Organisation's categories of "effectiveness, efficiency and satisfaction," the author determines ways in which usability can be quantified. 'Effectiveness' is the capacity for a product to generate (or enable) output; 'Efficiency' is measured as the level of effort invested in completing an action or task (for example, the author categorizes usability errors as distinguished between a 'slip' and a 'mistake' in user performance and experience. For him, a slip is when a user accidentally performs the wrong action which is readily corrected by the user, whereas a mistake is when a user thinks he is doing the right thing (intuitive action), but is unable to perform his task.
