Dr. mccall found that student performance was poor when students worked in groups compared to when each student worked on an individual project. in the context of group influence, this difference in performance can be best explained with the concept of:

social loafing

Explanation:

Social loafing:-

It is the concept in social psychology which refers to the phenomenon of a person exerting less amount of effort on a particular task if he/she is in a particular group when compared to the stance when he work alone.

It believes that working in groups helps in improving the quality of the work being accomplished as it will be a amalgamation of the different and unique skills and the talents of each individuals present in the group.

Hence, from the question, the difference of about Dr. McCall found is best explained by the concept  of social loafing.