If I were designing the experiment, I would have the control group be to play a different game instead of having it be maths instructions.
You generally don't want test subjects to know whether they are in the control condition or not. So if you're going to make it be maths instructions, you probably shouldn't tell them what the experiment is designed to test at all, until you're debriefing at the end. If you tell people you are recruiting that you are testing the effects of playing computer games on statistical reasoning, then the people in the control condition won't need to realize that what you're really testing is whether your RPG in particular helps people think about statistics. They can just play HalfLife 2 or whatever you pick for them to play for a few minutes, and then take your tests afterwards.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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