Imagine that you are an experimenter interested in whether a certain type of vitamin can improve working memory performance. You randomly assign participants to one of two groups. In one group they take the vitamin before completing a working memory task. In the other group they take a placebo (sugar pill) before completing the working memory task. When providing instructions to each group you emphasize to the vitamin group that they will be completing a task that requires a lot of concentration. When providing instructions to the placebo group, you don’t say anything about the level of concentration required of the task. What is the confounding variable?