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I am conducting an experiment to see if electric shock will make 8th graders be quiet. I fit half the room with electric shock collars and half the room does not get the collars. The students with collars get a shock every time they talk and the non collar students get told to be quiet. I keep a record to see which group talks the most. It turns out that one group is sitting next to a very noisy fan. This experiment is not really valid. This is because...
What is we changed more than one variable?