Hallway testing is a quick and cheap method of usability testing in which randomly-selected people—e.g., those passing by in the hallway—are asked to try using the product or service. This can help designers identify "brick walls", problems so serious that users simply cannot advance, in the early stages of a new design. Anyone but project designers and engineers can be used (they tend to act as "expert reviewers" because they are too close to the project).