In the days before computers, when all calculations were performed by hand, and even when computers were less powerful than they are today, you would check the significance level of a result by looking up the value in a table that listed the critical value of a given statistic for 5%, 1%, etc. For example, the critical value for the Student's t distribution with 10 degrees of freedom at a 5% significance level is 2.23. Suppose we performed a t test comparing the heights of 6 people from Cardiff and 6 from London (df = 6+6-2 = 10): if the t value were greater than this critical value of 2.23 we would conclude that London and Cardiff heights were different at the 5% significance level.
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