Design and evaluation happen iteratively one after the other. Why: to check that users can use the product and that they like it. What: a conceptual model, early prototypes of a new system and later, more complete prototypes. When: throughout design; finished products can be evaluated to collect information to inform new products. Iterative design, with its repeating cycle of design and testing, is the only validated methodology in existence that will consistently produce successful results. If you don"t have user-testing as an integral part of your design process you are going to throw buckets of money down the drain . See asktog. com for topical discussions about design and evaluation. Relies on observations from real use/exhibition of. Usability study (also: user study, usability testing, user testing) In a controlled lab environment to test usability aspects (e. g. efficiency, effectiveness of interface) In the field to test appropriation of the interface (i. e. whether users will use it in their lives.