Maureen is a psychology major who is conducting a study on memory. The participants in her experiment study a long list of words and then recall the list by verbalizing the words to Maureen. Maureen knows which words are correct, and whenever a participant states an incorrect word, Maureen inadvertently shifts her eyes and makes a note on her notepad. As a result, the participants change their recall behavior and tend to recall fewer words thereafter. This scenario illustrates a(n) ________.

Placebo effect
Experimenter effect
Correlation coefficient
Double-blind study