ABSTRACT
When researchers know the population mean and standard deviation for a given summary statistic they can compare a value for the statistic which has been obtained from one person or a sample of people with the population mean for that statistic, using a z-test. In this way, they can see how common the value they have obtained is among the population and thus how likely the person or group is to have come from a population with that mean and standard deviation. When only the population mean is known for the statistic a t-test has to be employed rather than a z-test.
The present chapter has largely concentrated on statistical significance as a way of deciding between a research hypothesis and a Null Hypothesis. In other words, it has only addressed the probability of making a Type I error (rejecting the Null Hypothesis when it is true).
