A psychologist, studying pilot trainees, picks a select group of trainees who are hopefully representative of all other trainees. The group of trainees being studied by this psychologist are collectively known to researchers as a .

Sample

Explanation:

In terms of research and experiment, sample is defined as group of individuals or participant, objects or other components. Samples tends to represent the large components, population etc in form of research sample which undergoes measurement.

According to the question, psychologist who is using certain group of pilot trainees for studying and research is depicting research sample representing numerous or large pilot trainee communities.

answer; archaeologists;

answer; opinion leaders;