<P> If the data - set of a randomly selected representative sample is very unlikely relative to the null hypothesis ( defined as being part of a class of sets of data that only rarely will be observed ) , the experimenter rejects the null hypothesis concluding it ( probably ) is false . This class of data - sets is usually specified via a test statistic which is designed to measure the extent of apparent departure from the null hypothesis . The procedure works by assessing whether the observed departure measured by the test statistic is larger than a value defined so that the probability of occurrence of a more extreme value is small under the null hypothesis ( usually in less than either 5 % or 1 % of similar data - sets in which the null hypothesis does hold ) . </P>
the value in the null hypothesis comes from ​ what
defined so that the probability of occurrence of a more extreme value is small under the null hypothesis ( usually in less than either 5 % or 1 % of similar data - sets in which the null hypothesis does hold )