Fred's advisor noticed that he had not included the same words in each list used to test whether the color of ink affects memory.This meant the lists differed in both content and ink color.Fred won't be able to easily interpret his results because:

Question

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The experiment included a confounding variable, which is the difference in content between the lists used to test the effect of ink color on memory. This makes it difficult to interpret whether any observed differences in memory performance between ink colors are due to the ink color itself or to the differing content of the lists.

Fred's Advisor Noticed That He Had Not Included the Same