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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).

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