Does your usability lab need a one-way mirror?

I've moderated UX research in over a dozen usability labs in the US, Germany and England and also in another dozen conference rooms. I've helped build the usability labs for both HumanCentric and the corporate headquarters for Truist Financial Corporation. Sometimes a fancy lab is essential and sometimes it's better to use a simpler room. This article will help you decide if you need a one-way mirror or not.

If you decide to install a one-way mirror, you should read my article, Building a one-way mirror.

What is a one-way mirror?

A one-way mirror is glass that has been coated with a thin layer of metal so that it's reflective, like a mirror. One side works as a mirror and the other side as a window. The terms one-way and two-way mirror are both used and mean the same thing. One-way mirrors are commonly used in usability labs and focus group rooms, but also in other locations like observation rooms in police stations, day care facilities, and reality TV filming locations.

Advantages of a one-way mirror

Capture of interactions - you can see all of the emotions on the face of the participant, their body language, and their interactions with both the product and moderator.
Wide view - you can see the entire room and session context, and are not limited to places that the camera can see. For studies that involve large products or moving around the room, camera views can be very restrictive. For example, we worked with Whirlpool, whose lab is a full kitchen with a mirrored wall; watching someone move around the kitchen through a camera is just not the same.
Makes you focus - there is something compelling about watching actual people that captures your attention; with video, is is easier to get distracted from the session. Although the huge flat-panel TV screens you can get today are more captivating than smaller screens used to be.
No equipment needed - once the mirror is installed, no equipment is necessary to use it, which means no equipment to break or software to crash during a session, and training for the moderator is minimal.
Showcase - one-way mirrors look cool and are great for tours.

Advantages of no mirror

Flexible study room - no need to orient toward the mirror location, or dedicate a wall of the room to a mirror.
Mirrors make people nervous - some participants notice the mirror and ask about observers behind it, although most quickly forget about it, especially if we have the flexibility to face them away from the mirror. To be fair, you can mitigate this by placing mini-blinds or sheer curtains in front of the mirror, and the cameras in rooms without a mirror make people almost as nervous as the mirror.
Flexible view - your point of view can be from anywhere you can place a camera, although you can set up similar observation monitors even with a one-way mirror.
No dedicated observation room - there is no need to dedicate a room as an observation room when you are watching via video streaming. This is great when space is at a premium. Our study rooms do get used as meeting rooms, but our observation rooms are only used for one purpose.
Lit observation room - you can keep the lights on for taking notes or having a meeting, or sit in a room with windows that aren't blocked by blackout curtains. This helps keep the observers awake, and makes them more likely to interact with each other and less likely to multi-task.
Sound proofing - it is difficult to design a one-way mirror that is as sound proof as a wall, and even if you do sound may still travel between adjacent rooms. If you have rowdy observers or your observers want to have a working meeting during the sessions, you can pick a room that is physically distant from the study room so participants cannot hear. I once had a participant say "Bless you" after hearing and observer sneeze. The design teams I work with enjoy observing in a space separated from the study room because they can have a lot more fun in the observation room.
Lit devices - a brightly lit device can easily be seen by participants. I've had difficulty with the lit apple on a MacBook, the lit "i" in the ThinkPad logo, smartphones, smart watches, etc.

Do you have to have a one-way mirror?

No, you do not have to have a one-way mirror.

One-way mirrors are most common where people want to show off, like a corporate or market research facility. They are very expensive and take up a lot of room. If you don't have the budget or the space, don't worry about it.

What to do instead?

Video conferencing
Mounted or tripod cameras
Video splitters
Watching on a personal laptop
Watching in a conference room with a big TV
Watching a recording later