Jane is in a study that aims to evaluate the effects of a new vitamin drink on energy levels. She is told that she will be in the group receiving the drink and is asked to rate her energy levels at different points throughout the day. She feels very energized by the drink and rates her energy levels high. At the end of the study, she is debriefed and told that the experiment involved deception. She was actually in the group that received only flavored water. What is this type of effect called, and how could it be prevented in a future experiment wishing to evaluate the vitamin drink?