Understanding Critical Value: Types and Calculations

Understanding Critical Value: Types and Calculations

The critical value plays a crucial role in various fields, especially statistics and hypothesis testing. It is a point of reference used to determine the significance of results obtained from experiments or analyses.

A critical value is a tipping point that helps researchers decide whether their findings are statistically significant or occurred purely by chance. It is derived from probability distributions and varies based on factors like sample size, desired confidence level, and the specific statistical test used.

This article will explain the following Basic concepts of critical value:

  • Definition of Critical value
  • The formula of critical value
  • Types of critical value
  • Examples to calculate the critical values

Let’s cover all the above concepts and try to gain more insight into critical value.

The Meaning of Critical Value:

A critical value refers to a specific point or threshold in statistical analysis that helps determine the acceptance or rejection of a null hypothesis. It is a value used in hypothesis testing to assess whether there is enough evidence to support a claim about a population parameter.

When test statistics (like t-tests or z-tests) exceed the critical value then it suggests that the results are unlikely to have occurred by chance and lead to the rejection of the null hypothesis in favor of an alternative hypothesis.

Formula to Compute Critical Value:

The critical value in statistics is not determined by a single formula.  It depends on various factors such as:

·       The significance level (α)

·       Degrees of freedom (df)

·       The specific statistical test being conducted (e.g., t-test, z-test, chi-square test).

Categories of Critical value:

Critical values can vary depending on the statistical test being conducted and the significance level chosen for hypothesis testing. Here are different types or categories of critical values often encountered in statistical analysis:

1.   Z-Critical Values:

·       Related to the standard normal distribution (z-distribution).

·       Used in hypothesis testing when dealing with population means and known standard deviations.

Steps to calculate:

1.     Establish the alpha level.

2.     Determine the complementary value of the significance level (α) by subtracting α from 1 for a two-tailed test

3.     In the case of a one-tailed test find out the complementary probability by subtracting the significance level (α) from 0.5.

4.     Find the corresponding z-score or critical value by referencing the z-distribution table. When conducting a left-tailed test, if the critical value is obtained for the left side of the distribution, simply include a negative sign to this value to signify the tail direction for the test.

2.   T-Critical Values:

·       Associated with the t-distribution.

·       Utilized hypothesis testing when dealing with population means and unknown standard deviations, commonly used for small sample sizes.

Steps to calculate t critical value:

1.     Find out the alpha level

2.     Get a df by subtracting 1 from the sample size.

3.     If the hypothesis test relates to a single direction, consult the one-tailed t-distribution table. In contrast, when dealing with a test that considers both directions, refer to the two-tailed t-distribution table.

4.     Locate the degrees of freedom (df) on the left side of the table and the alpha value (α) on the top row. Identify the point where these values intersect within the table to determine the t critical value.

3.   Chi-Square Critical Values:

·       About the chi-square distribution.

·       Used in hypothesis testing for goodness-of-fit tests, tests of independence, and tests of homogeneity.

Steps to calculate:

1.     Decide the significance level (alpha).

2.     Compute the df using the formula: df = n - 1.

3.     Refer to the chi-square distribution table.

4.     Find the intersection point of alpha and df in the table.

5.     Retrieve the chi-square critical value from this intersection in the table.

4.   F-Critical Values:

·       Corresponding to the F-distribution.

·       Employed in hypothesis testing for comparing variances or testing the equality of means of more than two groups.

Steps to calculate:

1.     Decide the alpha level.

2.     Minus one from the size of 1st group. This defines the first degree of freedom as x.

3.     Do the same previous step for the 2nd group and say y.

4.     Referring to the F distribution table, locate the f critical value by identifying the point where the x column intersects with the y row within the table.

Examples to Calculate the Critical Value:

These examples demonstrate how we calculate the critical value by different tests.

Example 1:

Calculating Z-Critical Value Suppose a researcher wants to conduct a hypothesis test with a significance level (α) of 0.19 for a two-tailed test. Find the z-critical value(s).


1.     α = 0.05 for a two-tailed test, so the complementary value is 1 - α = 1 - 0.19 = 0.81.

2.     Locate the z-critical value for a 0.81 probability in the standard normal distribution table.

3.     From the table, the z-critical value for a 0.81 probability is approximately ± 0.8779

Example 2: T-Critical Value

Given a sample size of 9 and α = 0.05 for a two-tailed test, find the t-critical value.


1.     Degrees of freedom (df) = sample size - 1 = 9 - 1 = 8.

2.     Refer to the t-distribution table for df = 8 and α = 0.05 to determine the t-critical value. ± 2.306

3.     Locate the degrees of freedom (df) on the left side of the table and the alpha value (α) on the top row. Identify the point where these values intersect within the table to determine the t critical value which is ± 2.306.

Final words:

This article explored critical values in statistics and hypothesis testing. They vary based on factors like significance level and sample size. Z-critical, T-critical, Chi-square critical, and F-critical values are crucial for distinct tests.

Calculating critical values involves specific steps for each statistical test. For instance, determining the z-critical value requires referencing the standard normal distribution table, while the t-critical value relies on the t-distribution table. 

Courtesy : CBSE