**Confidence
Interval for the Mean - Population Standard Deviation Unknown
**

**Brief Instructions**

Press
*STAT*, choose *TESTS* and then *TInterval...* and
press *ENTER*. Next to Inpt: choose *Stats*
and press *ENTER*. Then enter the
correct numbers next to the corresponding symbols. *C-Level* stands for *Confidence
Level*. Put the cursor on *Calculate*
and press *ENTER*. The confidence
interval is displayed in correct form just below *TInterval*. You may need to round
the endpoints to a reasonable number of decimal places.

**Detailed Instructions**

To demonstrate this procedure, suppose we have a sample mean of 43.7, a sample standard deviation of 6.2, a sample size of 25, and we want 90% confidence in our result.

Press
*STAT*, choose *TESTS* and then choose *TInterval...* and
press *ENTER*. You will see the *TInterval* input screen. Next to *Inpt**:* choose *Stats* by putting the cursor on *Stats*
and pressing the *ENTER* button.

Now
enter the correct numbers next to the corresponding symbols. To enter a number, move the cursor to the
right of the symbol using the arrow buttons and press the buttons for the
digits adding decimal points or minus signs if necessary. *C-Level*
is short for *Confidence Level*.

When you have finished, the numbers should appear as shown below.

_{}

If your
numbers are in the correct position, put the cursor on *Calculate* and press *ENTER*. You should see the following display.

_{}

Since the sample mean has only one place accuracy in this case, the confidence interval is approximately (41.58, 45.82).