Final answer:
The design used by the professor is called a Pretest-Posttest Design, as it evaluates changes in the same group of students' critical thinking skills before and after the lectures.
Step-by-step explanation:
The professor's approach to testing the effectiveness of his lectures on students' critical thinking by administering a test at the end of the semester is an example of a Pretest-Posttest Design. This design involves assessing the same group of students' knowledge or abilities before and after exposure to a treatment, which in this case is the lecture series on critical thinking. It allows the professor to measure the changes in the students' critical thinking skills over the course of the semester. Unlike cross-sectional or correlational designs, the pretest-posttest design is more focused on observing changes in the same subjects over time. A longitudinal design involves multiple tests over a much longer timeframe, while a correlational design would only assess the relationship between two variables without a pre-established treatment.