A statistic is calculated from a sample. To begin with, we assume that the hypothesis about the population parameter is true. We compare the value of the statistic with the hypothetical value of the parameter. If the difference between them is small the hypothesis is accepted, and if the difference between them is large the hypothesis is rejected. A statistic that is used to decide whether to accept or reject a hypothesis is called a test-statistic. It is important to remember that a test-statistic does not prove the hypothesis to be correct, but it acts as evidence against the hypothesis. Some of the test-statistics to be discussed later are Z, t and Chi – Square.