Who should facilitate?
An interesting question is, who should perform the testing and report on the results?
The designer is most familiar with the design--the research that leads to the concepts and ideas being tested. The designer is also most familiar with the product's target audience and user personas. The design being tested is the result of thinking that has emerged from that material. Who then, is better qualified to plan, facilitate, and analyze the results of usability testing?
Usability tests are often scripted, asking the participant to follow certain paths of interaction that map out to high-priority tasks. The tester is asked to think aloud and share with the facilitator their thinking, responses, and rationale for departing from the expected response, or for interacting with the design as expected.
When the designer facilitates such tests, the designer is attuned to opportunities to depart from the script, in response to user comments and behavior that was not expected or that...