Indeed, what Tom Sloper said.
You may want to study psychology and social methods for studying, for example your test subjects can't receive help from you AT ALL. If they fail to install the game, you can't help them. If they fail to open the game, you can't help them. If they get stuck on a level, don't interrupt them to give tips. Even if they ask you and insist, you have to say "it's up to you" (but record the question, it's very likely they're confused by something that needs to be addressed). Even if they failed to do something as simple as installing the program or get to the first level from the main menu, there's a 95% chance it's your fault and has to be fixed.
Additionally, you may want to include statistics gathering in your game. For example FPS games record areas where players get killed more often; you can record average life meter, length to pass each level, areas where player spend most/less time, deaths per match/level/etc. Whatever applies to your game.
For example if they spend too much in an area, it may be because it's poorly balanced (an area having too much to explore), players get stuck on something, it's a hard level, its layout looks like there's a hidden/hard to reach area (even though there's none; aka it looks more interesting than it should be), etc, etc... you get the idea.