The halo effect is when an initial good impression influences the way one views other aspects of a system.
The halo effect can be a problem during evaluation as one aspect of a system under consideration, often the surface appearance, may mean that users rate other more critical aspects, such as ease of use, based on the initial impression. This can mean that a prototype needs to be far more well developed than strictly necessary. Using a more sketch-like low-fidelity prototype might help, depending on the kinds of issue being investigated.
Used in glossary entries: low-fidelity prototype, prototype, sketch