How to Find the T Critical Value on a TI-84 Calculator


When you conduct a t-test, you will get a test statistic as a result. To determine if the results of the t-test are statistically significant, you can compare the test statistic to a T critical value. If the absolute value of the test statistic is greater than the T critical value, then the results of the test are statistically significant.

To find the T critical value on a TI-84 calculator, we can use the following function:

invT(probability, v)

where:

  • probability: the significance level
  • v: the degrees of freedom

You can access this function on a TI-84 calculator by pressing 2nd and then pressing vars. This will take you to a DISTR screen where you can then use invT():

inverse T distribution on a TI-84 calculator

This tutorial shares several examples of how to use the invT() function to find T critical values on a TI-84 calculator.

Example 1: T Critical Value for a Left-Tailed Test

Question: Find the T critical value for a left-tailed test with a significance level of 0.05 and degrees of freedom = 11.

Answer: invT(.05, 11) = -1.7959

T critical value for left-tailed test on TI-84 calculator

Interpretation: If the test statistic of the t-test is less than -1.7959, then the results of the test are statistically significant at α = 0.05.

Example 2: T Critical Value for a Right-Tailed Test

Question: Find the T critical value for a right-tailed test with a significance level of 0.05 and degrees of freedom = 24.

Answer: invT(1-.05, 24) = 1.71088

T critical value for a right-tailed test on a TI-84 calculator

Interpretation: If the test statistic of the t-test is greater than 1.71088, then the results of the test are statistically significant at α = 0.05.

Example 3: T Critical Value for a Two-Tailed Test

Question: Find the T critical value for a two-tailed test with a significance level of 0.05 and degrees of freedom = 13.

Answer: invT(.05/2, 13) = -2.1604, 2.1604

T Critical value for a two-tailed test on a TI-84 calculator

Interpretation: Since this is a two=tailed test, we actually have two critical values: -2.1604 and 2.1604. If the test statistic of the t-test is less than -2.1604 or greater than 2.1604, then the results of the test are statistically significant at α = 0.05.

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