Final answer:
The research method used by Dr. Sears observing behavior in a department store is naturalistic observation, a manner of studying behaviors in their own environment without intervention.
Step-by-step explanation:
The research method used by Dr. Sears to observe that the more time a person spends in a department store, the more purchases they tend to make, can best be described as naturalistic observation. This method involves watching behaviors in their natural setting without intervention or manipulation by the researcher. The aim is to see how people act in real-world settings, which in this case, is within the context of a department store environment.
Naturalistic observation is a part of observational research that can require significant investments of time and money, and it often relies on the fortunate occurrence of the researched events. Dr. Sears's study merely records the behaviors and amount of time spent in the setting without trying to influence those behaviors, which is a hallmark of naturalistic observation.