Lila doesn't like her psychology class because the instructor uses unannounced pop exams to test the class. As a result, she never knows when she will be tested. Her instructor is testing her on a schedule.

variable-interval    

Explanation:

Variable-interval schedule: In psychology, the term "variable-interval schedule" is described as a schedule of reinforcement that occurs in the operant conditioning theory. In the variable-interval schedule, a specific reinforcement is being given to a particular response only after a specific time has been passed (unpredictable time), yet this time is being kept on a variable/changing schedule.

In the question above, Lila instructor is testing her on a variable-interval schedule.

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