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The chaotic, messy, and uncontrollable factors from the real world that invade a controlled experiment and ruin its beautiful, clean data. They are everything the researchers didn't think of, couldn't control for, or actively ignored to make their study publishable. If a lab study shows that people in a quiet room perform better on puzzles, the External Variables are the screaming kids, the pounding hangover, and the constant phone notifications of real life. They are the reason why a drug that works in 100% of carefully selected mice only works in 30% of chaotic, genetically diverse, cheeseburger-eating humans.
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