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Phrase(s): Garbage in, garbage out.
Prov. If you give nonsensical instructions to people or computers, those instructions will produce nonsensical results. • Jill: Why is my computer generating all this gibberish? Jane: You must have made a mistake in the program. Garbage in, garbage out. • Ed insists that children are so ignorant nowadays because their teachers are incompetent. "Garbage in, garbage out," he says.
McGrawhill's American Idioms And Phrasal Verbs