Dana is recording playground behavior of students to investigate the differences between third, fourth, and fifth graders. For each session, Dana sits on the swings with a notebook and counts the size of groups and types of activities for each grade. She notices that several children in each grade come over to the swings, but then choose to walk away once they notice Dana. Because they walked away, they are not counted in the study. Dana finds that the lower grades tend to do more group activities, while the higher grades tend to be more individualistic. Which problem is affecting this quasi-experiment?

Based on the information provided within the question it seems that the problem that is affecting this quasi-experiment is the observer effect. In the context of psychology this term refers to a theory stating that observing a phenomenon or experiment may change the results. Which is what is happening in this situation since Dana's presence alone seems to causing the children to not do what they normally do. Thus affecting the experiment.