Level 1 Inspection : review by two experts against common usability rules of thumb (i.e. 'heuristics')
Usability Inspection is used to make a quick assessment of the quantity and magnitude of commonly-observed usability problems. Because actual users or consumers are not part of Usability Review, it must never be the sole criterion on which the goodness of a product is based.
It is critical to keep in mind at all times, that real users always turn up surprises that are missed by the Reviewers.
Level 2 Expert Review: review by at least 4 experts against a comprehensive list of usability guidelines
Expert Review is used to make a quick assessment of the quantity and magnitude of commonly-observed usability problems. Because actual users or consumers are not part of Expert Review, it must never be the sole criterion on which the goodness of a product is based.
It is critical to keep in mind at all times, that real users always turn up surprises that are missed by the Experts.
Level 3 Expert Review: review by 12 -16 (combination of usability experts and domain experts). The larger number of reviewers allows statistically valid assertions (but still preliminary) to be made about the product.
Level 4 Usability Test: formal test involving 12–16 actual or potential users
Level 5 Usability Test: A scientific comparison of two versions or two different products
