Critical Value – Formula, Definition With Examples
Created on Dec 27, 2023
Updated on January 8, 2024
In the fascinating world of statistics, the concept of a Critical Value is a fundamental cornerstone. As pioneers in child education, we at Brighterly understand the importance of introducing complex concepts like this one in a manner that’s engaging and easy to comprehend for our young learners. In fact, we’ve always believed that a firm grasp of statistics can be an invaluable tool in a child’s academic journey, paving the way for a deeper understanding of the world around them.
A critical value is what we use in a hypothesis test to decide whether an observed effect or result is statistically significant. In other words, we use it to determine if our result is due to chance or if there’s a statistically significant difference at play. It’s a crucial element in various fields such as research, economics, psychology, and naturally, mathematics. This blog post will take you on a journey of understanding what a critical value is, how it’s used, its key properties, and how it differs from another vital concept – the test statistic. We’ll also explore the formula for calculating critical values and even have some practice problems for our Brighterly learners to tackle!
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What Is a Critical Value?
A Critical Value serves as the threshold in a hypothesis test in statistics. It’s a crucial concept that children learning statistics should understand because it’s used in various fields, such as economics, psychology, and scientific research. Here at Brighterly, we believe that children who understand statistical concepts such as critical values from an early age will have an advantage in their academic journey.
When we perform a hypothesis test, we compare a test statistic to a critical value. If the test statistic is more extreme than the critical value, we reject the null hypothesis. For example, suppose we’re testing a new teaching method at Brighterly, and we hypothesize that it improves math scores. We could compare the average scores of students who used the method (the test statistic) to a critical value. If the average scores are significantly higher, we can conclude that the teaching method is effective.
Definition of Critical Value in Statistics
In the field of Statistics, a critical value determines the dividing line between the region where we reject the null hypothesis and where we fail to reject it. In simpler terms, it acts as a marker that indicates when an outcome is unusual or significant.
An excellent analogy would be to imagine you’re throwing darts at a target. The critical value is the boundary that separates the ‘bullseye’ (the region where you’d be surprised to land a dart) from the rest of the target. Anything landing within that boundary is considered ‘significant’. So, in the context of our Brighterly teaching method example, a significant result means that our new method appears to be effective.
Definition of Test Statistic
In the realm of statistics, a Test Statistic is a mathematical formula that allows us to decide whether to reject the null hypothesis. It’s the value we compare to the critical value in our hypothesis test.
Consider it as the actual ‘dart’ we throw on our imaginary statistical dartboard. In our Brighterly example, the test statistic was the average math score of the students who used our new teaching method. We calculate the test statistic based on our sample data.
Properties of Critical Values
Critical values have some key properties that make them a valuable tool in statistics.

Fixed by significance level: The value of the critical value depends on the significance level that we choose for our test. A common significance level is 5% (0.05), but it can be any value that the researcher decides.

Depends on the distribution: Critical values depend on the type of probability distribution that we’re using. Commonly used distributions in statistics include the normal distribution, tdistribution, and chisquare distribution.

Directional: Critical values can be onetailed (checking for an effect in one direction) or twotailed (checking for an effect in both directions).
Properties of Test Statistics
Test statistics also have specific properties that make them essential in statistics.

Calculated from sample data: A test statistic is calculated using the data from our sample. It reflects the data we have collected.

Depends on the null hypothesis: The way we calculate the test statistic depends on our null hypothesis – the statement that we’re testing.

Random: Because test statistics are calculated from sample data, they are random variables. This means that if we collected a different sample, we would likely get a different test statistic.
Difference Between Critical Value and Test Statistic
Both critical value and test statistic play significant roles in hypothesis testing, but they differ in several ways. The critical value is a predetermined threshold at which we reject the null hypothesis, while the test statistic is calculated from the sample data.
Think of the critical value as the ‘goal post’, and the test statistic as the ‘ball’. The aim of our statistical ‘game’ is to see if the ‘ball’ (test statistic) can go beyond the ‘goal post’ (critical value).
Formulas for Critical Values
Formulas for calculating critical values vary depending on the nature of the statistical test and the distribution involved. For instance, for a hypothesis test involving a normal distribution, the critical value (z*) can be found using the Ztable. For a tdistribution, you would use the ttable.
Understanding the Formula for Calculifying Critical Value
A good grasp of the formula for calculating the critical value can further students’ understanding of statistics. For a Ztest, the critical value can be found from the Ztable, which shows the relationship between Zscores (a measure of how many standard deviations an element is from the mean) and percentages.
For a Ttest, the critical value comes from the Ttable. The degrees of freedom (df), which are related to the sample size, and the chosen significance level, determine the critical value.
Writing the Formula for Critical Values
Writing down the formula might look a bit different depending on the statistical test used. For a Ztest, the formula for the critical value (Z*) can be written as Z = Z(1α/2)* for a twotailed test, where α is the chosen significance level.
For a Ttest, the formula could be written as *T = T(df, 1α/2)**, where df is the degrees of freedom, and α is the chosen significance level.
Practice Problems on Calculating Critical Values
Now, let’s apply what we have learned with some practice problems. This would help reinforce the concept and formula for critical values.

What is the critical value for a onetailed Ztest with a significance level of 5%?

Find the critical value for a twotailed Ttest with 10 degrees of freedom and a significance level of 5%.

Calculate the critical value for a onetailed Ttest with 15 degrees of freedom and a significance level of 1%.
Remember, practice makes perfect!
Conclusion
In the realm of statistics, understanding critical values is of paramount importance. They provide us with a statistical threshold that allows us to make significant conclusions and decisions in various fields. Our exploration of critical values in this post aimed to provide a simplified understanding of these complex statistical concepts. As always, our mission here at Brighterly is to make learning fun, engaging, and simple for all our young learners.
Understanding statistics not only equips children with the tools to explore more advanced mathematical concepts but also nurtures their analytical thinking and problemsolving abilities. It’s exciting to imagine the possibilities that open up when children are not just consumers of statistical information but also understand the processes that underpin this information. We hope that the concept of critical values and their significance is a little clearer after reading this post. Remember, every complex journey begins with a simple step, and we’re here to walk each of these steps with you at Brighterly. Happy learning!
Frequently Asked Questions on Critical Values
What is a critical value?
A critical value is a key component in hypothesis testing in statistics. It’s a threshold or cutoff point that we use to decide whether we reject or fail to reject our null hypothesis. When we perform a hypothesis test, we compare our test statistic to the critical value. If the test statistic is more extreme than the critical value, then we reject our null hypothesis. Essentially, it’s like the finish line in a race, where the runners are different statistical outcomes.
What’s the difference between a critical value and a test statistic?
While both the critical value and test statistic play significant roles in hypothesis testing, they have different functions. A critical value is essentially the line of demarcation in a hypothesis test. It’s a threshold value that we compare our test statistic to. On the other hand, the test statistic is the result that we get from our sample data. Think of the critical value as the ‘goal line’ and the test statistic as the ‘ball’. The aim of our statistical ‘game’ is to see if the ‘ball’ (test statistic) can go beyond the ‘goal line’ (critical value).
How do I calculate a critical value?
The method of calculating a critical value varies depending on the type of statistical test you’re performing. For instance, if you’re performing a Ztest, you would use a Ztable, which links Zscores to percentages, to find your critical value. For a Ttest, you would use a Ttable. The Ttable lists critical values for Ttests based on the degrees of freedom (which is related to your sample size) and your chosen significance level. It’s important to note that using these tables often requires a good understanding of the concepts of probability and distribution, which are foundational concepts in statistics.