Section 5.5: One-tailed vs. Two-tailed Tests

Fundamentals of Social Statistics by Adam J. McKee

Recall that the null hypothesis is a foundational concept in statistics. It essentially represents a default position, suggesting no effect or no difference. To draw conclusions from experiments or studies, we assess whether there’s enough evidence to reject this null hypothesis.

Test Statistics and Probability Distribution

When deciding to accept or reject the null hypothesis, researchers rely on the test statistic—a single value derived from sample data. Its position within an associated probability distribution becomes crucial. A probability distribution, in its simplest form, is a function that provides the probabilities of occurrence of different possible outcomes. Rejecting the null hypothesis occurs if the test statistic is deemed sufficiently improbable. This improbability is determined by observing if the result’s probability falls within the extreme ends of the distribution, known as tails. These tails are defined by a parameter called alpha (α).

Two-Tailed vs. One-Tailed Tests

When analyzing data, it’s essential to decide whether to use a one-tailed or two-tailed test, as each has its own implications and uses.

Two-Tailed Test

In the realm of statistical analysis, the alpha level represents a predefined threshold that determines the point at which the null hypothesis can be rejected. When researchers choose an alpha of 0.05, or 5%, they’re establishing a clear boundary for making a significant decision based on the resulting data. This choice means that there’s a 5% risk of rejecting the null hypothesis when it’s actually true. Specifically, they would be poised to reject the null hypothesis if the test statistic they compute from their sample data resides in the extreme 2.5% of either end of a probability distribution. This approach inherently acknowledges the presence of both tails, which are the extreme ends of a normal distribution curve.

The concept of “tails” in a distribution is essential as they encompass the regions containing the most extreme values. When delving into the specifics of these tails in relation to the alpha level, standard deviation units become a pivotal measure. Standard deviation, in essence, quantifies the variability or dispersion within a set of data points. When an alpha of 0.05 is set, the critical value for rejecting the null hypothesis in a normal distribution is 1.96 standard deviations away from the mean or central value.

Therefore, any test statistic that is either 1.96 standard deviation units to the left or right of the mean becomes a candidate for significant results, leading to the rejection of the null hypothesis. This particular criterion ensures that only the most extreme and unlikely outcomes, under the assumption that the null hypothesis is true, are deemed significant. The balance struck by this method ensures a systematic approach to hypothesis testing, guarding against rash decisions based on random variations in data.

One-Tailed Test

In statistical hypothesis testing, there are two main types of tests that researchers might employ: one-tailed and two-tailed tests. The decision to use a one-tailed test arises when there’s a specific, theoretically-driven expectation about the direction of a relationship or difference. For instance, a researcher might predict that one group will score significantly higher than another, rather than just predicting a difference without specifying its direction. By employing a one-tailed test, researchers essentially focus their attention on one end or “tail” of the probability distribution. This means they’re looking exclusively for outcomes that are exceptionally high (or low) in a predetermined direction. A significant benefit of this approach is that it consolidates the entire 5% rejection region – often denoted by an alpha level of 0.05 – into just one of the distribution’s tails. This increases the test’s sensitivity or power in that particular direction, making it more adept at detecting effects that align with the initial hypothesis.

Contrasting this with a two-tailed test helps underscore the differences. In a two-tailed scenario, researchers remain open to extreme outcomes in either direction, effectively splitting the alpha level between both tails of the distribution. Practically, in a standard distribution, this means that a test statistic would have to surpass 1.96 standard deviations from the mean to reject the null hypothesis. However, when employing the one-tailed approach, this criterion becomes less stringent, requiring the test statistic to exceed only 1.65 standard deviations in the specified direction.

This seemingly small shift in threshold has significant implications. The lowered threshold in one-tailed tests, represented by the 1.65 standard deviation mark, renders it easier to achieve statistical significance and hence reject the null hypothesis. However, this increased power comes with a trade-off: the researcher must be correct in their directional prediction, or the results become invalid. The choice between these tests underscores the importance of theoretical grounding in statistical analyses, ensuring results are both significant and meaningful in the context of the research question.

Implications of Using One-Tailed Tests

One-tailed tests, in the realm of statistical hypothesis testing, certainly come with an allure. At a glance, their heightened sensitivity or “power” makes them a compelling choice for researchers aiming to detect specific directional effects in their data. This increased power essentially means that, under a one-tailed test, it’s comparatively easier to find statistically significant results if they align with the anticipated direction. However, this advantage is not without its strings attached. A critical tenet of the one-tailed test is the requirement for an “a priori” specification. This Latin term, translating to “from the earlier,” emphasizes the necessity for researchers to specify, right at the outset, the expected direction of the difference in means they anticipate between groups or conditions.

But why is this upfront specification so crucial? The reason lies in the very nature of a one-tailed test. By design, it’s tailored to detect effects in one specific direction. Therefore, if researchers were to observe a difference in the opposite direction to what they predicted, even if this difference is substantial, it would be dismissed as statistically insignificant under the rules governing one-tailed tests.

This stringent requirement underscores the importance of having a strong theoretical basis when opting for a one-tailed test. Researchers must be reasonably confident in their directional hypothesis. The consequences of an incorrect prediction render any observed effects in the unanticipated direction moot, emphasizing the importance of careful consideration and foresight in the choice and application of statistical tests.

Considerations in Using One-Tailed Tests

Statistical tests provide researchers with a systematic approach to assess their hypotheses using data. When it comes to determining the significance of results, the directionality of tests becomes crucial. Directionality refers to whether a statistical test is one-tailed (testing an effect in a specific direction) or two-tailed (testing an effect in either direction).

While the increased power of one-tailed tests might seem like an appealing choice, they come with specific requirements and considerations. A significant stipulation is the need for a priori specification. Before conducting the analysis, researchers must determine the expected direction of the mean differences. If observed results diverge in the opposite direction from expectations, the findings are deemed non-significant, regardless of their statistical strength. Such an approach ensures that researchers don’t capitalize on chance findings and that hypotheses are well-defined and grounded in theory or previous evidence.

However, the principle of directionality does not universally apply to all statistical tests. In the domain of hypothesis testing, the t-test is a prominent example where researchers can choose between one-tailed or two-tailed tests based on their research questions. But not all tests have this flexibility.

Take the F-test, typically employed in analysis of variance (ANOVA) or regression analysis. Unlike t-tests, F-tests are inherently non-directional. A few reasons underpin this:

  • The F-distribution, the basis for the F-test, is exclusively right-skewed and consists of only positive values. Unlike the t or z distributions, there’s no negative side, which means there’s no ‘left tail’ and ‘right tail’ in a conventional sense.
  • The primary role of the F-test is to compare variances (in ANOVA) or to evaluate the overall fit of a model (in regression). The test doesn’t measure the ‘direction’ of an effect, but its existence. For instance, in ANOVA, an F-test determines if group means are different from one another but doesn’t specify which group is higher or lower.
  • Therefore, when interpreting F-values, researchers infer differences (in ANOVA) or model fit (in regression) without a directional context.

In essence, while the concept of directionality is vital in some statistical tests, it’s not a one-size-fits-all rule. Depending on the nature and goals of the test, researchers may or may not have the luxury to hypothesize and test directional effects.


The null hypothesis in statistics posits that there’s no significant effect or difference. To evaluate this, researchers examine a test statistic derived from sample data, which is then contextualized within a probability distribution—a function detailing the likelihood of possible outcomes. If this statistic is found in the extreme ends or “tails” of this distribution, it can warrant rejecting the null hypothesis.

Choosing between a one-tailed or two-tailed test is vital in this process. A two-tailed test examines both ends of a distribution, with an often-used alpha level of 0.05 indicating a 5% risk of wrongly rejecting the null hypothesis. For such a test, a result must be more than 1.96 standard deviations from the mean (in either direction) to be considered significant. Conversely, a one-tailed test focuses on just one direction of effect, requiring outcomes to be beyond 1.65 standard deviations in a predetermined direction to be significant. This approach offers greater sensitivity but demands a clear, prior hypothesis about the direction of the effect. If results oppose this initial prediction, they’re deemed insignificant.

However, not all statistical tests fit this mold. The F-test, used in ANOVA and regression analysis, inherently lacks directionality. It’s based on the right-skewed F-distribution, which only has positive values. The F-test’s role is not about pinpointing the direction of an effect but confirming its presence. As such, researchers don’t decide on directionality when using it, illustrating that the notion of one or two “tails” isn’t universally applicable across all statistical procedures.

Key Terms

Hypothesis, Sample, Population, Generalization, Inference, Test Statistic,  Research Hypothesis, Null Hypotheses, p-values, Alpha Level, Type I Error, Type II Error, Power, Assumptions, One-tailed Test, Two-tailed Test

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Last Modified:  09/25/2023

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