It is possible to manipulate different variables associated with a prototype and study their effects precisely. For example, if different interface styles are to be compared, users may be asked to undertake a series of tasks with the different interfaces which are carefully matched for difficulty. All other aspects of the situation are controlled, for example speed of response of the system, other hardware and software used or lighting conditions. Any differences in performance can then be reliably attributed to the difference in interface. In addition, laboratory evaluations produce exact measures to guide the further design of a product.
Contextual evaluations tend to have lower reliability.
The validity of such evaluations depends on the purpose of the evaluation. Laboratory evaluations support development of a working prototype, but contextual evaluations support development of a working product by showing what really happens in the field. For example, laboratory evaluation of communication software may assess if users are able to select a symbol from a selection set, whereas in contextual evaluation additional aims of the evaluation may be to investigate whether a communication device can be used for discussions or if users increase their social circles or improve their discussion skills. Contextual evaluations are highly valid from rehabilitative point of view because they give feedback from those real situations which AT is intended to target. In addition, contextual evaluation raises service related problems which retailers may face and serves as a way for industrial partners to learn about the necessary product related services.