How to Run a Successful User Test

TL;DR
This part of the Lean User Tests series focuses on selecting and inviting interviewees: Who is a suitable candidate, how to invite them and why you will need replacement candidates.
You should now be around three to four weeks away from your next—or first—user test. If anything is impeding you from reaching that goal, please do not hesitate to contact me by commenting this post, and I will gladly help you.
Thoughts on the Right Interviewees
In the B2C scenario mentioned above, we were mostly interested in user segments:
Multiples buyers: These customers had purchased items on at least two occasions within six months prior to attending the user test.
Prospective first time buyers: These users were either registered or had subscribed to one or more newsletters, but had not yet purchased anything.
In the case of the multiple buyers, the interview was supposed to determine what would make them buy more. And the second group was particularly interesting, as the company spent considerable amounts of money on acquiring new customers. These customer acquisition investments would only make sense, if these new clients actually end up buying multiple times in the future.
Number of interviewees and observers during interviews:
Contrary to the usual set-up—separating the interview room from the observers—, we chose the one-room model:
In a large meeting/conference room the interviewers and interviewees sat on one side of the table, while the person recording the protocol and the observers sat on the other side at the table.
This process proved to be beneficial, because on the one hand it requires a lot less logistics and is therefore much more cost efficient. On the other hand, the observers have a greater chance to participate in the communication during the interviews.
The two-room model facilitates this only via indirect communication, which usually causes a time delay and is prone to misunderstandings. A direct participation of the observers, however, requires that they are qualified and especially trained, so they don’t highjack the interview with their interrupting questions.
During these interviews, the maximum number of individuals taking part additionally to the interviewee should not exceed four to five people. This seems to be the most successful combination and so far has not caused any problems during any interview.
The interview circle is therefore made up as follows:
Interviewer: She leads the interviewee according to a script through the interview.
The user or interviewee answers the questions of the interviewer and solves tasks that are given to her.
Recording person: She supports the interviewer during the evaluation of the interview afterwards through the written recording of the interview and the most important answers of the interviewee.
Observer 1 belongs to the circle of stakeholders, follows the interview and takes notes. She is allowed to ask questions for a better understanding.
Observer 2: See above.
Observer 3: See above. (Mostly, a third observer would be considered too much already.)
It has been established that it is pretty much impossible to lead the interview and at the same time record all the feedback. This is the reason why there should always be a recording person solely for this purpose.
It is also recommended that the recording person and the interviewer switch roles during the afternoon, if the recording person has been trained accordingly. While the role of the interviewer requires a lot of experience and a good knowledge of the test subject, the role of the recording person–limited to taking notes—can easily be filled by one of the observers.