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Confidence Interval for the Mean - Population Standard Deviation Known

Brief Instructions

Press STAT, choose TESTS and then ZInterval... 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 ZInterval. 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 58.1, a population standard deviation of 2.3, a sample size of 40, and we want 95% confidence in our result.

Press STAT, choose TESTS and then choose ZInterval... and press ENTER. You will see the ZInterval input screen.

Next to Inpt: choose Stats by putting the cursor on Stats and pressing the ENTER button. Then 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 enter the number in the usual fashion. 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, your approximation for the confidence interval should be (57.39, 58.81) .

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