What’s the difference between an informal usability review and an expert usability review?

An informal usability review can be conducted by anyone, including stakeholders or developers, and does not require formal usability training or qualifications. It is typically subjective and based on general impressions. In contrast, an expert usability review (also called heuristic evaluation) is conducted by a trained usability expert who applies recognized usability principles. This is what differentiates the two approaches most clearly. Options A and C are misleading; expert reviews can be informal in format, and informal reviews aren't necessarily based solely on opinion. Option D is incorrect since both informal and expert reviews can be conducted individually or in groups.