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For example, consider a clinical trial of a drug which is thought to reduce the risk of death. However, it may be found that the drug actually ''increases'' the risk of death. If H<sub>0</sub> was defined as 'there is no difference in risk of death according to treatment status', then H<sub>1</sub> would be 'there is a difference in risk of death according to treatment status', and a two-tailed test would be performed. This hypothesis test would be expected to find evidence against the null hypothesis (the direction of which would be quantified through a measure of effect such as the risk ratio). However, if the investigators were convinced that the drug would not increase the risk of death, then H<sub>0</sub> may be stated as 'the risk of death is not reduced amongst treated animals', in which case H<sub>1</sub> would be 'the risk of death is reduced amongst treated animals'. In this case, a one-tailed hypothesis test would be performed, which would fail to find evidence against the null hypothesis (since the null is in fact correct). For this reason, two-tailed tests are used in the vast majority of cases.
For example, consider a clinical trial of a drug which is thought to reduce the risk of death. However, it may be found that the drug actually ''increases'' the risk of death. If H<sub>0</sub> was defined as 'there is no difference in risk of death according to treatment status', then H<sub>1</sub> would be 'there is a difference in risk of death according to treatment status', and a two-tailed test would be performed. This hypothesis test would be expected to find evidence against the null hypothesis (the direction of which would be quantified through a measure of effect such as the risk ratio). However, if the investigators were convinced that the drug would not increase the risk of death, then H<sub>0</sub> may be stated as 'the risk of death is not reduced amongst treated animals', in which case H<sub>1</sub> would be 'the risk of death is reduced amongst treated animals'. In this case, a one-tailed hypothesis test would be performed, which would fail to find evidence against the null hypothesis (since the null is in fact correct). For this reason, two-tailed tests are used in the vast majority of cases.
[[Category:Veterinary Epidemiology - Statistical Methods|E]]
[[Category:Veterinary Epidemiology - Statistical Methods|F]]