To control for experimenter bias as well as extraneous variables such as severity of condition, attitudes, and age, researchers use assignment in establishing their experimental and control groups.

random                                            

Explanation:

Random assignment: In scientific or psychological research, the term random assignment is defined as the utilization of chance procedures in psychological research pr experiments to make sure that every subject or participant involved in a particular study gets the equal opportunity of getting assigned in any of the group, for example, treatment group or experimental group, etc.

In the question above, the given statement states the use of random assignment.

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