One of the most fundamental usability methods is, of course, user testing, but the lack of time and resources sometimes makes this approach unfeasible in a corporate setting. Problems also arise and compound the difficulty of achieving relevant test results when we include user testing with an iterative design process. Therefore, one must differentiate between formative evaluation and summative evaluation.* Formative evaluation is performed in conjunction with steps of an iterative design process. Summative comes at the end and evaluates the overall quality of a design. This later evaluation type is most commonly used.
* From Jakob Nielsen's Usability Engineering
another version of the blueprint graphic.