Probability and Statistics > TI 89 > T Distribution on TI 89

## 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.

Very NIce . Can u elucidete on what u mean by sigma is not known ?

Umesh,

There are just some cases where you don’t know the standard deviation of a population sample (sigma).

Stephanie

Wow. This is really terrible. What kind of instructions say “press ENTER ENTER” There is no indication of what I’m pressing ENTER on. I tried it. It didn’t work. Now how do we figure out what I did wrong? You can’t, because I don’t even know what I was SUPPOSED to be pressing ENTER on. Absolutely horrible. Tutorials aren’t supposed to be mechanical, they should tell the decisions you are making, not just what buttons you are literally punching. I’ve never seen a worse “tutorial”. Beyond useless.

It sounds like you don’t have the STAT/LIST editor installed on your calculator? If you don’t have it, then pressing ENTER twice won’t work. I added that to the step to clarify that you’re entering the editor.