A researcher wishes to study the effectiveness of a tutoring program for middle school students. She divides a group of 50 students into two groups of 25, making sure that the 25 students with the lowest grades receive tutoring. At the end of the term she looks at the average change in grade for each group. What important aspect of her experimental design has she neglected to consider?

Select one: a. replication b. placebo effect c. control group