# One-Tailed Test vs. Two-Tailed Test: What's the Difference?

Edited by Aimie Carlson || By Harlon Moss || Published on March 14, 2024
A one-tailed test assesses the direction of an effect (greater or less than), while a two-tailed test evaluates the existence of any effect, regardless of direction.

## Key Differences

A one-tailed test is used when a researcher has a specific hypothesis about the direction of an effect or relationship. A two-tailed test, conversely, is appropriate when the researcher does not have a directional hypothesis and is open to finding an effect in either direction.
In a one-tailed test, the significance is checked in one direction of the distribution, either greater than or less than a certain value. In a two-tailed test, the significance is evaluated on both ends of the distribution, accounting for both possible directions of the effect.
One-tailed tests are often more powerful than two-tailed tests for detecting an effect in a specified direction because they focus the statistical testing power in one tail of the distribution. However, two-tailed tests are more conservative and are used when the direction of the effect is not predetermined.
Researchers use a one-tailed test when they hypothesize that a parameter will either increase or decrease, but not both. A two-tailed test is chosen when the hypothesis admits the possibility of an effect in either direction, making it a safer choice when the direction of the effect is uncertain.
The choice between a one-tailed and a two-tailed test affects the interpretation of statistical significance. In a one-tailed test, a significant result supports the hypothesis in one specific direction, while in a two-tailed test, it indicates a significant effect but does not specify the direction.

## Comparison Chart

### Direction of Effect

Tests for effect in one specified direction
Tests for effect in either direction

### Significance Evaluation

In one end (tail) of the distribution
In both ends (tails) of the distribution

### Hypothesis Specificity

Requires a specific directional hypothesis
Does not require a directional hypothesis

### Statistical Power

More power to detect effect in one direction
Evenly distributes power in both directions

### Suitability

Used when the direction of the effect is known or hypothesized
Used when direction of effect is unknown or could be either

## One-Tailed Test and Two-Tailed Test Definitions

#### One-Tailed Test

Assumes the null hypothesis is false in one direction.
Using a one-tailed test, researchers found an increase in customer satisfaction after service improvements.

#### Two-Tailed Test

Does not require a specific directional hypothesis.
The two-tailed test was appropriate to see if there was any difference in effectiveness between the two drugs.

#### One-Tailed Test

Tests a hypothesis in a specific direction.
A one-tailed test was used to determine if the new drug significantly lowers blood pressure.

#### Two-Tailed Test

Tests both ends of the probability distribution.
A two-tailed test showed significant change in reaction times, but did not specify the direction of change.

#### One-Tailed Test

Focuses on one end of the probability distribution.
The one-tailed test showed significant increase in sales after the marketing campaign.

#### Two-Tailed Test

Equally sensitive to effects in both directions.
Using a two-tailed test, researchers found significant changes in patient outcomes, both improvements and declines.

#### One-Tailed Test

More powerful in detecting effects in a specified direction.
The one-tailed test revealed a significant decrease in cholesterol levels due to the diet.

#### Two-Tailed Test

Suitable when unsure of the effect direction.
A two-tailed test was chosen to investigate whether changing the diet influences body weight, regardless of whether it increases or decreases.

#### One-Tailed Test

Suitable for hypotheses predicting an increase or decrease.
A one-tailed test was conducted to check if employee training reduced the error rate.

#### Two-Tailed Test

Assesses the possibility of an effect in either direction.
The two-tailed test was used to evaluate if the new teaching method had any effect on test scores.

## FAQs

#### What is a two-tailed test?

A statistical test that evaluates the existence of an effect in either direction, without specifying the direction.

#### What is the advantage of a two-tailed test?

It's more conservative, allowing for the detection of effects in either direction.

#### Does a one-tailed test have more statistical power?

Yes, in the specified direction, because it concentrates the statistical significance in one tail.

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

It's a statistical test that assesses the direction of an effect or relationship in one specific direction.

#### When is a one-tailed test used?

When the researcher has a specific hypothesis about the direction of an effect, such as expecting an increase or decrease.

#### Can a one-tailed test detect effects in both directions?

No, it only tests for an effect in one predetermined direction.

#### Is a two-tailed test more suitable when the direction of effect is unknown?

Yes, because it assesses both possible directions of the effect.

#### Can the choice of test affect research outcomes?

Yes, choosing the wrong test type can lead to incorrect conclusions about the data.

#### How does hypothesis specificity differ between these tests?

A one-tailed test requires a specific directional hypothesis, while a two-tailed test does not.

#### How does the interpretation of results differ between these tests?

In a one-tailed test, results are interpreted in the context of the specified direction, while in a two-tailed test, the direction is not specified.

#### Can a two-tailed test be used to confirm a one-tailed hypothesis?

Yes, but it's less powerful for that specific hypothesis direction.

#### How does sample size affect these tests?

Larger sample sizes generally provide more reliable results for both test types.

#### What if the actual effect is in the opposite direction in a one-tailed test?

The one-tailed test would fail to detect this, as it only looks in one direction.

#### Can you switch from a two-tailed to a one-tailed test after seeing the data?

No, this is considered inappropriate as it can lead to biased results.

#### What is the null hypothesis in a one-tailed test?

It states that there is no effect or a lesser effect in the specified direction.

#### What does a significant result in a one-tailed test indicate?

It indicates a significant effect in the specified direction.

#### What is the null hypothesis in a two-tailed test?

It states that there is no effect in either direction.

#### Are one-tailed tests appropriate for exploratory research?

Typically no, because they require a specific direction for the hypothesis.

#### Are one-tailed tests less common than two-tailed tests?

Yes, because they require a strong justification for the directional hypothesis.

#### How do you decide which test to use?

The decision is based on the research question and hypothesis specificity.