How do you know if it is a one or two tailed test?

As a statistical expert with extensive experience in data analysis and hypothesis testing, I often come across the question of whether to perform a one-tailed or two-tailed test. The distinction between the two is crucial as it determines the direction of the alternative hypothesis and the corresponding critical region for decision-making.
Step 1: Understanding the Hypotheses
When conducting a hypothesis test, we start with the null hypothesis (H0), which is a statement of no effect or no difference. The alternative hypothesis (H1 or Ha), on the other hand, is what we believe to be true if the null hypothesis is false. The choice between a one-tailed and two-tailed test hinges on the nature of the alternative hypothesis.
**Step 2: Defining the Alternative Hypothesis**
- One-tailed test: This is used when we are interested in testing for a difference in a specific direction. The alternative hypothesis is directional, meaning it predicts that the sample statistic (e.g., mean) is either greater than or less than the population parameter (e.g., μ), but not both. For example, H1: μ > μ0 or H1: μ < μ0.
- Two-tailed test: This is used when we are interested in testing for a difference in any direction. The alternative hypothesis is non-directional, meaning it predicts that the sample statistic is different from the population parameter, without specifying the direction. For example, H1: μ ≠ μ0.
Step 3: Consider the Research Question
The decision between one-tailed and two-tailed tests should be guided by the research question or objective. If the question implies a specific direction (e.g., "Does the new drug increase the cure rate?"), a one-tailed test is appropriate. If the question is open to any difference (e.g., "Does the new drug affect the cure rate?"), a two-tailed test is more suitable.
**Step 4: Statistical Significance and Type I Error**
The level of statistical significance (α) is the same for both tests but is applied differently. In a one-tailed test, all the α is on one side, whereas in a two-tailed test, α is split between the two tails. This affects the likelihood of committing a Type I error, which is the probability of rejecting the null hypothesis when it is actually true.
Step 5: Power of the Test
The power of the test, or the probability of correctly rejecting a false null hypothesis, is also a consideration. A one-tailed test has more power to detect an effect in the specified direction but is less sensitive to effects in the opposite direction.
**Step 6: Conventional Practices and Guidelines**
In many fields, there are conventions or guidelines that suggest the use of one or two-tailed tests. For instance, in pharmaceutical research, it's common to use one-tailed tests for specific claims about efficacy. In exploratory research, a two-tailed test is often preferred to remain open to all possible outcomes.
Step 7: Consulting with a Statistician
Given the complexity of the decision and its impact on the interpretation of results, consulting with a statistician can be beneficial. They can provide insights based on the specific context of the study and the potential implications of the test choice.
In conclusion, the decision to use a one-tailed or two-tailed test is not one to be taken lightly. It requires careful consideration of the research question, the nature of the alternative hypothesis, the implications for Type I error, and the power of the test. It is a decision that will significantly influence the outcome and interpretation of the statistical analysis.

For example, we may wish to compare the mean of a sample to a given value x using a t-test. Our null hypothesis is that the mean is equal to x. A two-tailed test will test both if the mean is significantly greater than x and if the mean significantly less than x.