Null Hypothesis (from Internet Glossary of Statistical Terms)

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Null Hypothesis
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The null hypothesis is a term that statisticians often use to indicate the statistical hypothesis tested. The purpose of most statistical tests, is to determine if the obtained results provide a reason to reject the hypothesis that they are merely a product of chance factors. For example, in an experiment in which two groups of randomly selected subjects have received different treatments and have yielded different means, it is always necessary to ask if the difference between the obtained means is among the differences that would be expected to occur by chance whenever two groups are randomly selected. In this example, the hypothesis tested is that the two samples are from populations with the same mean. Another way to say this is to assert that the investigator tests the null hypothesis that the difference between the means of the populations from which the samples were drawn, is zero. If the difference between the means of the samples is among those that would occur rarely by chance when the null hypothesis is true, the null hypothesis is rejected and the investigator describes the results as statistically significant.