In this case, the researcher is predicting either an increase or a decrease as a result of treatment, but not both. In this case the null hypothesis is rejected only if the sample data fall in the predicted tail of the distribution. Even when a particular direction of treatment effect can be predicted a two-tailed test is still often used. The goal of hypothesis testing is usually to correctly reject the null hypothesis, or in other words, to show that the treatment applied has had an effect. The probability of correctly rejecting the null hypothesis is called the power of a statistical test.
Statistical power is large when the treatment effect is large.
Put another way, you are more likely to correctly reject the null hypothesis when the treatment has created a large difference between your sample data and the original population. Statistical power is greater for one-tailed tests. Larger samples provide greater power. Finally, read the summary on pp. This is a personal page published by the author.