Nolo's Plain-English Law Dictionary

Burden Of Proof

A party's job of convincing the decisionmaker in a trial that the party's version of the facts is true. In a civil trial, it means that the plaintiff must convince the judge or jury by a preponderance of the evidence that the plaintiff's version is true -- that is, over 50% of the believable evidence is in the plaintiff's favor. (That said, the burden of proof may shift to the defendant if the defendant raises a factual issue in defense to the plaintiff's claims.) In a criminal case, because a person's liberty is at stake, the government has a harder job, and must convince the judge or jury beyond a reasonable doubt that the defendant is guilty.
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