T Distribution on TI 89: Overview
When you have a small sample size, or don’t know the standard deviation of a population, you can’t use the central limit theorem (which says that certain statistics like the sample mean will be normally distributed as long as you have a large enough sample size). In these cases, you need to use a T score, not a Z score. Unlike the Z scores, there isn’t one table: there are many, and it depends upon the number of “degrees of freedom” (the number of independent observations) in your data set. For most T-distribution questions in elementary stats, you’ll usually be given all of the information you need to plug into the calculator and retrieve the T score. You might be asked to find the area under a T curve, or (like Z scores), you might be given a certain area and asked to find the T score.
T Distribution on TI 89 Steps
Note: You need to have the STAT/LIST editor installed for these procedures. You can download a copy for free from the TI-website.
Sample problem: Find the area under a T curve with degrees of freedom 10 for P( 1≤ X ≤ 2 ).
Step 1: Press APPS.
Step 2: Press ENTER twice to enter the STATS/LIST Editor.
Step 3: PressF5 for F5Distr.
Step 4: Choose 6 for 6:t Cdf.
Step 5: Enter 1 in the box for Lower Value.
Step 6: Enter 2 in the box for Upper Value.
Step 7: Enter 1 0 in the box for Deg of Freedom, df.
Step 8: Press ENTER. This returns the result .133753.
Sample problem: find the T score with a value of 0.25 to the left and df of 10.
Step 1: Press APPS.
Step 2: Press ENTER twice to enter the STAT/LIST Editor.
Step 3: Press F5 for F5Distr.
Step 4: Press 2 for Inverse.
Step 5: Press the right arrow button.
Step 6: Press 2 for Inverse t and then press ENTER.
Step 7: Enter 0.25 in the Area box.
Step 8: Enter 10 in the Deg of Freedom, df box.
Step 9: Press ENTER. The calculator returns the result of -.699812.
Tip: For P( X ≥ a ), enter 10^99 in the box for Upper Value and for P(X ≤ b), enter 10^99 in the box for Lower Value.
That’s how to find a T Distribution on TI 89!
Lost your guidebook? Download on from the TI website.