# One-Tailed Test Explained: Definition and Example

Summary:

Hypothesis testing in statistics involves determining whether a claim about a population parameter is true or not. One of the fundamental concepts in this realm is the one-tailed test, which focuses on establishing whether a sample mean is significantly higher or lower than the population mean, but not both. This article delves into the definition and applications of one-tailed tests, providing a clear example from the world of finance. Discover how financial analysts employ this statistical tool to assess investment hypotheses with confidence.

## What is a one-tailed test?

A one-tailed test is a statistical hypothesis test designed to show that the sample mean is either significantly higher or lower than the population mean, but not both. Unlike a two-tailed test that assesses changes in both directions, a one-tailed test focuses on just one direction of interest. It derives its name from examining one side or “tail” of a probability distribution, typically a normal distribution, although it can apply to other distributions as well.

### Setting up null and alternative hypotheses

Before conducting a one-tailed test, researchers must establish null and alternative hypotheses. The null hypothesis (H0) represents the claim they aim to challenge, while the alternative hypothesis (Ha) is the assertion supported by rejecting the null hypothesis. A one-tailed test is sometimes referred to as a directional hypothesis or directional test.

Weigh the risks and benefits

Here is a list of the benefits and drawbacks of one-tailed tests:

##### Pros
• Efficient for testing specific directional relationships.
• Focuses analysis on one side of the distribution, eliminating the need to account for the opposite direction.
• Useful when researchers are only interested in changes in one direction.
##### Cons
• May overlook significant changes in the opposite direction.
• Not suitable when you want to examine changes in both directions.

## Example of the one-tailed test

Imagine an analyst seeking to demonstrate that a portfolio manager has outperformed the S&P 500 index by 16.91% in a given year. They may frame the null and alternative hypotheses as follows:

### Determining significance in a one-tailed test

To assess the significance of the difference in returns, a significance level (often denoted as p) must be specified. The significance level represents the probability of incorrectly concluding that the null hypothesis is false. Commonly used significance levels are 1%, 5%, or 10%, but other values can be chosen as needed. A lower p-value indicates stronger evidence against the null hypothesis.

If the resulting p-value is less than the chosen significance level, typically 5%, the analyst can confidently reject the null hypothesis. In our example, if the p-value is 0.03 (3%), the analyst can be 97% confident that the portfolio outperformed the market index, supporting their claim.

### When to use a one-tailed test

A one-tailed test is appropriate when you are solely interested in assessing changes in one direction and can disregard changes in the opposite direction. In our example, the analyst is concerned with whether the portfolio’s return is greater than the market’s and does not need to account for the possibility of underperformance. In situations where it’s crucial to test both directions, a two-tailed test should be employed.

## How to determine if it’s a one-tailed or two-tailed test

In essence, a one-tailed test checks for an increase or decrease in a parameter in one specific direction. Conversely, a two-tailed test looks for changes in both directions, either an increase or a decrease.

## When should you use a two-tailed test?

A two-tailed test should be chosen when you want to examine changes in your hypothesis in both directions, ensuring a comprehensive assessment.

## Frequently asked questions

### What is a one-tailed test used for?

A one-tailed test is used to investigate the possibility of a change in one specific direction while disregarding changes in the opposite direction. It is employed when researchers are interested in assessing only one side of the distribution.

### How do you determine if it’s a one-tailed or two-tailed test?

A one-tailed test is chosen when you want to focus on changes in one direction, while a two-tailed test is selected when you need to consider changes in both directions.

### Why might someone prefer a one-tailed test over a two-tailed test?

Researchers might prefer a one-tailed test when they have a specific directional hypothesis and are not concerned with changes in the opposite direction. It can provide more power to detect effects in that one direction.

### What is the significance level in a one-tailed test?

The significance level, often denoted as “p,” represents the probability of incorrectly concluding that the null hypothesis is false. Commonly used significance levels are 1%, 5%, or 10%, but other values can be chosen based on the research context.

### How is a one-tailed test different from a two-tailed test in terms of statistical power?

A one-tailed test can have higher statistical power than a two-tailed test when researchers have a specific directional hypothesis. This means it’s better at detecting effects in the chosen direction but may not detect effects in the opposite direction.

### What are some real-world examples of when to use a one-tailed test?

One-tailed tests are used in various fields. For instance, in finance, analysts may use a one-tailed test to determine if an investment outperforms a benchmark by a specific margin. Similarly, in medicine, researchers might use it to test if a new drug is more effective than an existing treatment.

## Key takeaways

• A one-tailed test examines whether a sample mean is significantly higher or lower than a population mean in one specific direction.
• It is efficient for testing specific directional relationships and is suitable when changes in one direction are of primary interest.
• Researchers must establish null and alternative hypotheses before conducting a one-tailed test.
• Significance levels (p-values) are used to determine the strength of evidence against the null hypothesis, with common choices being 1%, 5%, or 10%.
• A one-tailed test is employed when it is not important to test the outcome in the opposite direction.
###### View article sources
1. Introduction to Hypothesis Testing – University of Notre Dame
2. Government Social Research Example Knowledge Test – Gov.uk
3. One tailed hypothesis tests. – University of Huston