Critical Values > T Critical Value
Not sure what I mean by a “critical value”? You might want to read this article first: What is a Critical Value?
What is a T Critical Value?
The t-distribution (black line) is taller than the normal (dotted line). Image: Columbia.edu
You’ll use your t value in a hypothesis test to compare against a calculated t score. This helps you to decide if you should support or reject a null hypothesis.
How to Find a T Critical Value
You’ve got several options for finding a T value with technology, including:
- TI 83: See “How to Find t Critical Value on the TI 83.”
- Excel: See Excel T Test; Excel will calculate the T critical value as part of the process.
By hand
Step 1: Subtract one from your sample size. This is your df, or degrees of freedom. For example, if the sample size is 9, then your df is 8 – 1 = 7.
Step 2: Choose an alpha level. The alpha level is usually given to you in the question — the most common one is 5% (0.05).
Step 3: Choose either the one tailed T Distribution table or two tailed T Distribution table). This depends on if you’re running a one tailed test or two.
Step 4: Look up the df in the left hand side of the t-distribution table and the alpha level along the top row. Find the intersection of the row and column. For this example (7 df, α = .05,) the t crit value is 1.895.
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