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Confidence Interval for the Population Proportion

Brief Instructions

Press STAT, choose TESTS, choose 1-PropZInt... and press ENTER. Enter the correct numbers next to the corresponding symbols. C-Level is short for Confidence Level. Choose Calculate and press ENTER. The confidence interval appears below 1-PropZInt in the correct format. You may have to round the endpoints to a reasonable number of decimal places.

Detailed Instructions

To demonstrate this procedure, suppose we have 34 successes in 80 trials and want 98% confidence in our result.

Press STAT, choose TESTS and then choose 1-PropZInt... and press ENTER. You will see the 1-PropZint input screen.

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 number of successes and the number of trials have at most 2 significant digits, the confidence interval should have three-place accuracy. So in this case, the confidence interval is approximately (0.296, 0.554).

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