Using operant conditioning is the act of using stimulus and reaction to begin training a response for your roommate.
The antecedent stimulus is the event or stimulus that precedes a response. What needs to happen to get the roommate to act is there needs to be some action that absolutely necessitates a response. Let's say for the sake of the example you plan a movie night in your room. This is the stimulus.
The response is obviously how your roommate will respond to this stimulus. Hopefully they will respond by cleaning up their part of the room. If they respond in another way, such as complaining or simply refusing to clean, then you can take different responses.
The reinforcement is your response to their action. Assuming the roommate cleaned up, you would encourage and thank them, perhaps mentioning that it'd be fun to do this sort of thing more often. This gives an incentive and encourages the roommate to do the right thing—as well as giving them a reward for having done it this...
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