Always start each new problem from the home screen

Lets say we want to calculate a **95 %** confidence interval
for a population mean from some sample data. The sample std. dev. is **6**
, and the sample mean is **55. **The sample size is **15.** In this problem
we should use a **T-interval **not a **Z-interval**. ( we have a sample
**< 30 **, we do **not** know the population standard deviation)

On the calculator go to the **"TESTS"** menu.
Press

**Arrow right** to "**TESTS**"
and down to **" 8 : TInterval "** (see screen below)

Press (this screen will appear)

Once again we must choose **"Stats". **Press

**Arrow down** and change
the settings on your screen to match these.

** X : 55 , Sx : 6 , n : 15 , C-Level : .95 **

**Highlight " Calculate" and press
(you will see this screen) **

As we can see from the screen above the **95%** confidence
interval is **(51.677 , 58.323)**. This means that from repeated samples of
this population will get a mean value between **51.677** an **58.323 95%**
of the time.

**Example 4 (****Using actual Sample Data)**

The duration (in minutes) for a sample of **18** **OSCAR
**telephone registrations is shown below: (begin by typing the data into **L1**)

**2.1 4.8 5.5 10.4
3.3 3.5 4.8 5.8 5.3
**

**2.8 3.6 5.9 6.6
7.8 10.5 7.5 6.0 4.5**

What is the point estimate of the population mean? Compute
the **98%** confidence interval for the population mean.

Again we need to use a **T-interval** since the sample
is less than **30 **and of unknown distribution with unknown sigma.

On the calculator we want to go to the **"TESTS"**
menu. So press

**Arrow right** to ** "TESTS"**
and down to **"8 : TInterval" **

Press

(this screen will appear)

Now we want to highlight **"Data"** and press

Now the only thing left to change is the "**C-Level
:". **Make sure you change **"C-Level :"** to **.98** then
highlight **"Calculate**" and press

(you will see this screen)

This shows us the **98%** confidence interval is **(4.178,
7.0109).** Once again this means that repeated samples of this population will
yield a mean value between **4.178 **and **7.0109 98%** of the time.