One-tailed Test vs. Two-tailed Test: Know the Difference
By Shumaila Saeed || Published on January 19, 2024
One-tailed test predicts a direction in results (higher or lower), while two-tailed test checks for any significant difference, irrespective of direction.
Key Differences
A One-tailed Test focuses on determining if there is a statistically significant difference in a specific direction (greater or lesser). In contrast, a Two-tailed Test evaluates if there's any significant difference in either direction, without a specific prediction.
Shumaila Saeed
Jan 19, 2024
The One-tailed Test is used when a hypothesis predicts a direction of the outcome. Conversely, the Two-tailed Test is applicable when the research does not predict the direction, instead just questioning the existence of a difference.
Shumaila Saeed
Jan 19, 2024
Sensitivity is higher in a One-tailed Test for detecting effects in one direction, as it concentrates statistical power in that direction. On the other hand, a Two-tailed Test splits the statistical power for both directions, being less sensitive but more comprehensive.
Shumaila Saeed
Jan 19, 2024
In a One-tailed Test, the significance level is all set in one tail of the distribution, making it more prone to type I errors if the prediction is wrong. The Two-tailed Test divides this level between both tails, reducing the risk of such errors when direction is not certain.
Shumaila Saeed
Jan 19, 2024
Researchers use the One-tailed Test when they have a strong theoretical foundation to expect a directional result. In contrast, the Two-tailed Test is preferable in exploratory or initial research where the direction of the effect is not anticipated.
Shumaila Saeed
Jan 19, 2024
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Comparison Chart
Direction of Hypothesis
Predicts a specific direction
Does not predict direction
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Jan 19, 2024
Sensitivity
More sensitive to effects in one direction
Less sensitive, but covers both directions
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Jan 19, 2024
Risk of Errors
Higher risk of Type I error if wrong direction is predicted
Lower risk as it covers both directions
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Jan 19, 2024
Application
Used with strong theoretical backing for direction
Used in exploratory research without directional hypothesis
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Jan 19, 2024
Significance Level Allocation
Entirely in one tail of the distribution
Divided between both tails of the distribution
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Jan 19, 2024
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One-tailed Test and Two-tailed Test Definitions
One-tailed Test
More powerful for detecting effects in a predicted direction.
A one-tailed test revealed a significant increase in sales post-advertisement.
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Jan 05, 2024
Two-tailed Test
Tests both sides of the mean in a distribution.
A two-tailed test determined if the medication altered heart rate either way.
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Jan 05, 2024
One-tailed Test
Focuses on one side of the mean in a distribution.
To check if the average weight is less than 150 lbs, a one-tailed test is applied.
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Jan 05, 2024
Two-tailed Test
Equally sensitive to effects in both directions.
A two-tailed test showed significant change in customer satisfaction, both increase and decrease.
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Jan 05, 2024
One-tailed Test
Assumes no significant effect in the non-predicted direction.
The one-tailed test ignored the possibility of decreased performance post-training.
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Jan 05, 2024
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Two-tailed Test
Allocates significance level across both tails of distribution.
The two-tailed test revealed an unexpected direction in the treatment’s effect.
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Jan 05, 2024
One-tailed Test
Used when the hypothesis has a directional prediction.
To determine if a training program improves scores, a one-tailed test was conducted.
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Jan 05, 2024
Two-tailed Test
Assesses for any significant difference, regardless of direction.
To see if a diet affects weight, a two-tailed test was used.
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Jan 05, 2024
One-tailed Test
A statistical test predicting a direction of difference.
In testing if a new drug increases recovery rate, a one-tailed test is used.
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Jan 05, 2024
Two-tailed Test
Applicable when hypothesis does not specify a direction.
Investigating if a new teaching method impacts grades, a two-tailed test was chosen.
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Jan 05, 2024
Repeatedly Asked Queries
When is a One-tailed Test used?
When the hypothesis has a directional prediction.
Shumaila Saeed
Jan 19, 2024
Can a One-tailed Test detect effects in non-predicted directions?
No, it only focuses on the predicted direction.
Shumaila Saeed
Jan 19, 2024
Is a One-tailed Test more sensitive than a Two-tailed Test?
Yes, but only in the predicted direction.
Shumaila Saeed
Jan 19, 2024
What are the risks of a One-tailed Test?
Higher risk of Type I error if the predicted direction is incorrect.
Shumaila Saeed
Jan 19, 2024
What is a One-tailed Test?
It's a statistical test predicting outcome in a specific direction.
Shumaila Saeed
Jan 19, 2024
What is a Two-tailed Test?
A test assessing for any significant difference, regardless of direction.
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Jan 19, 2024
Can a Two-tailed Test detect effects in both directions?
Yes, it's equally sensitive to effects in both directions.
Shumaila Saeed
Jan 19, 2024
Is a Two-tailed Test less powerful than a One-tailed Test?
Yes, in terms of sensitivity in a specific direction.
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Jan 19, 2024
Can a One-tailed Test miss significant effects?
Yes, if effects occur in the non-predicted direction.
Shumaila Saeed
Jan 19, 2024
Can a One-tailed Test become a Two-tailed Test?
Not directly; the test type should align with the hypothesis.
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Jan 19, 2024
Which test has a higher chance of detecting unexpected results?
Two-tailed Test, as it checks both directions.
Shumaila Saeed
Jan 19, 2024
Are One-tailed Tests suitable for testing non-specific hypotheses?
No, they are designed for specific, directional hypotheses.
Shumaila Saeed
Jan 19, 2024
Why choose a Two-tailed Test?
When the hypothesis does not specify a direction.
Shumaila Saeed
Jan 19, 2024
Is a Two-tailed Test more conservative than a One-tailed Test?
Yes, as it considers both directions equally.
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Jan 19, 2024
What type of error is more common in One-tailed Tests?
Type I error, due to the prediction of a specific direction.
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Jan 19, 2024
Are Two-tailed Tests common in exploratory research?
Yes, due to their non-directional nature.
Shumaila Saeed
Jan 19, 2024
Can a Two-tailed Test be used for directional hypotheses?
It can, but it's less efficient than a One-tailed Test for this purpose.
Shumaila Saeed
Jan 19, 2024
Should a beginner researcher use a Two-tailed Test?
Yes, it's a safer choice when unsure about the direction of the effect.
Shumaila Saeed
Jan 19, 2024
How does a Two-tailed Test allocate significance level?
It's divided between both tails of the distribution.
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Jan 19, 2024
Do Two-tailed Tests require more data than One-tailed Tests?
No, data requirements depend on the study, not the test type.
Shumaila Saeed
Jan 19, 2024
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About Author
Written by
Shumaila SaeedShumaila Saeed, an expert content creator with 6 years of experience, specializes in distilling complex topics into easily digestible comparisons, shining a light on the nuances that both inform and educate readers with clarity and accuracy.
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