What heuristic is demonstrated when Glen adjusts his GPA estimation based on midterm grades?

The heuristic demonstrated when Glen adjusts his GPA estimation based on midterm grades is anchoring. Anchoring occurs when an individual relies heavily on the initial piece of information encountered (in this case, the midterm grades) when making decisions or estimations. Glen's original GPA estimation serves as the anchor, and upon receiving new information from his midterm grades, he adjusts his estimation accordingly.

This behavior showcases how initial numerical values or information can influence subsequent judgments, often leading individuals to stay close to their original estimates even when new data is presented. The adjustment process may not fully consider all factors, but it relies on the starting point that was established, which is characteristic of the anchoring heuristic.