Get Critical Value Table

If you want the critical value of t for a two-tailed test, divide the significance level by two.

A critical value is a line on a graph that splits the graph into sections. One or two of the sections is the “rejection region“; if your test value falls into that region, then you reject the null hypothesis.

To find the critical value for an f test the steps are as follows: Find the alpha level. Determine the degrees of freedom for both samples by subtracting 1 from each sample size. Find the corresponding value from a one-tailed or two-tailed f distribution at the given alpha level. This will give the critical value.

For a 95% confidence interval Zcritical = 1.96. The problem states a sample size of 30, so we will use use t-critical with 30-1=29 degrees of freedom. A 95% confidence interval would encompass all but the bottom 2.5% and the top 97.5% which correspond to probabilities of 0.025 and 0.975.

How to find t critical value? Subtract 1 from the sample size to get the degree of freedom. Degree of Freedom = N – 1 = 5 – 1. Depending on the test, choose one-tailed t distribution table or two-tailed t table below. Look for the degree of freedom in the most left column.

In the t-distribution table, find the column which contains alpha = 0.05 for the one-tailed test. Then, find the row corresponding to 20 degrees of freedom. The truncated t-table below shows the critical t-value. The row and column intersection in the t-distribution table indicates that the critical t-value is 1.725.

In statistics, critical value is the measurement statisticians use to calculate the margin of error within a set of data and is expressed as: Critical probability (p*) = 1 - (Alpha / 2), where Alpha is equal to 1 - (the confidence level / 100).