Observers - Paper Prototyping: The Fast and Easy Way to Define and Refine User Interfaces - page 177

Working up to In-Room Observers
Is there a limit to how many observers you can have? It depends on several factors: the size of the
room, whether you're doing co-discovery, the user population, and the experience of the facilitator.
Cram six observers into a small room with one timid technophobe and the psychological discomfort will
be so palpable that you could cut it into blocks and build a retaining wall. But put those same six
observers in a larger conference room with a pair of confident domain experts and the session is likely
to be relaxed and fun. My rule of thumb is about half a dozen team members (including the Computer
and facilitator) per user, although sometimes I have allowed more. In practice it's rarely feasible to have
more than 10 observers because it's too difficult for them to see the prototype.
As Figure 10.1 shows, my position on in-room observers is at the upper end of the spectrum compared
with that of many of my peers. However, I have a decade of experience in training observers and in
helping users feel at ease. One way to get started is to have a small number of observers—one or
two—in the room, and the rest elsewhere. Start by inviting the people you think will be easiest to have
in there with you, and afterward have them compare their experiences with those of the people who sat
in the other room. People can take turns being in-room observers, and you can gradually increase the
limit if and when you are comfortable doing so.