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Definition of Null hypothesis
1. Noun. (statistics) A hypothesis set up to be nullified or refuted in order to support an alternative hypothesis. When used, the null hypothesis is presumed true until statistical evidence in the form of a hypothesis test indicates otherwise. Therefore, the null and the alternative hypothesis must be mutually exclusive and exhaustive. The use of the null hypothesis is controversial. ¹
¹ Source: wiktionary.com
Medical Definition of Null hypothesis
1. The assumption that any observed difference between two samples of a statistical population is purely accidental and not due to systematic causes. (05 Dec 1998)