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

By Tayyaba Rehman — Published on January 13, 2024
A one-tailed test checks for an effect in one direction; a two-tailed test checks both directions.

## Key Differences

The one-tailed test is a statistical test used to determine if there's a significant effect in a specific, predetermined direction. The two-tailed test, on the other hand, examines if there's a significant effect in either of the two possible directions without a predetermined notion.
In hypothesis testing, the one-tailed test is focused on either detecting a significant increase or decrease but not both. Conversely, the two-tailed test is versatile and checks for significant differences in both the positive and negative directions.
When researchers have a specific expectation or hypothesis about the direction of the effect, they might choose the one-tailed test. However, if they want to remain neutral and consider possibilities in both directions, the two-tailed test becomes the ideal choice.
The decision to use a one-tailed test often comes with a caution: it increases the risk of missing an effect in the untested direction. In comparison, the two-tailed test, being unbiased, is generally considered a safer and more common approach in many research scenarios.
Choosing between the one-tailed test and the two-tailed test isn't merely about statistical procedures but also about the nature of the research question. While the one-tailed test can provide more power to detect an effect in the expected direction, the two-tailed test ensures that unexpected outcomes in the opposite direction are also considered.

## Comparison Chart

### Directionality

Tests in a predetermined direction
Tests in both possible directions

### Hypothesis

Direction-specific (increase or decrease)
Neutral (either increase or decrease)

### Research Application

When a specific direction is hypothesized
When no specific direction is hypothesized

### Risk

Might miss an effect in the untested direction
Detects effects in both directions

### Power

More power if the effect is in the expected direction
Spread out power in both directions

## Compare with Definitions

#### One-Tailed Test

Increases the chance of type II error in the untested direction.
Some critics argue that our one-tailed test might've missed an opposite effect.

#### Two-Tailed Test

Ensures consideration of unexpected outcomes in both directions.
They opted for a two-tailed test to catch any unforeseen negative impacts of the intervention.

#### One-Tailed Test

Requires a predetermined expectation of the effect's direction.
Since we anticipated the intervention would reduce symptoms, a one-tailed test was used.

#### Two-Tailed Test

Examines for significant differences, either positive or negative.
Researchers used a two-tailed test to check any changes, either improvements or declines.

#### One-Tailed Test

Has more power in the tested direction.
To maximize power for detecting increased sales, the team used a one-tailed test.

#### Two-Tailed Test

A statistical procedure testing an effect in both possible directions.
Without a specific expectation about the drug's impact, a two-tailed test was appropriate.

#### One-Tailed Test

Focuses on either a significant increase or decrease.
Believing the new curriculum would improve grades, educators opted for a one-tailed test.

#### Two-Tailed Test

Commonly preferred due to its unbiased nature.
Given the uncertainty in outcome direction, most scholars advocate for a two-tailed test.

#### One-Tailed Test

A statistical procedure for testing an effect in a specific direction.
Given our hypothesis that the drug increases scores, we conducted a one-tailed test.

#### Two-Tailed Test

Neutral with respect to the direction of the effect.
To consider both benefits and drawbacks of the policy, a two-tailed test was conducted.

## Common Curiosities

#### Is one-tailed test more powerful than two-tailed test?

A one-tailed test has more power if the effect is in the expected direction but risks missing effects in the untested direction.

#### Why might researchers avoid using a one-tailed test?

They might avoid it due to the risk of missing effects in the untested direction.

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

A one-tailed test is used when there's a specific hypothesis about the direction of the effect.

#### Which test is more common in general research scenarios?

The two-tailed test is more commonly used because of its unbiased approach.

#### How do the tests differ in terms of power?

One-tailed tests are more powerful in the tested direction, while two-tailed tests spread out power in both directions.

#### Can a one-tailed test detect negative effects if testing for positive ones?

No, a one-tailed test will only test in the predetermined direction.

#### Is there a risk associated with using a one-tailed test?

Yes, there's a risk of missing an effect in the untested direction.

#### When unsure about the direction of the effect, which test is advisable?

The two-tailed test is advisable when unsure about the direction of the effect.

#### What's the fundamental difference between a one-tailed test and a two-tailed test?

A one-tailed test checks in one predetermined direction, while a two-tailed test checks in both directions.

#### What's the primary advantage of using a two-tailed test?

A two-tailed test remains unbiased by checking for effects in both directions.

#### Is the choice between the tests only about statistical procedures?

No, it also depends on the nature of the research question.

#### How do the two tests differ in hypothesis framing?

One-tailed test has a direction-specific hypothesis, while a two-tailed test is directionally neutral.

#### In which test is the risk of a type II error greater in the untested direction?

In the one-tailed test, the risk of a type II error is greater in the untested direction.

#### Which test might be more suitable for exploratory research?

The two-tailed test, due to its ability to detect effects in both directions.

#### How does the two-tailed test ensure unexpected outcomes are considered?

It tests in both directions, allowing for the possibility of unexpected outcomes.

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