In one research project, a scientist gave students subliminal cassette tapes and told them to play them for 2 months. after 2 months, she assessed whether the tapes the self-esteem of her participants and found that self-esteem had indeed risen! while she may have attributed this to the tapes, it is also possible that those students may have found college life to be manageable and thus experienced an increase in self-esteem for that reason. this demonstrates which principle of critical thinking?

I believe the answer is: Ruling out rival hypotheses

Rival hypothesis refers to two possible alternatives to explain a certain occurrence which would contradict one another. Ruling out rival hypotheses is really crucial in order to improve internal validity of the result, and usually carried out by using  control techniques (most important is random assignment).

imperial crisis leads to civil war.

roman forces withdraw from britain as goths attack rome.

odoacer topples romulus augustus

huns attack the empire and drive german peoples ahead of them.