An educational psychologist wishes to know the mean number of words a third g ad

Question

An educational psychologist wishes to know the mean number of words a third g ader can read per minute. She wants to ensure that the estimate has an error of at most 0.24 words per minute. A previous study found that the mean was 29.7 words per minute. Assuming that the standard deviation is 3.7 what is the minimum number of third graders that must be included in a sample to construct the 99% confidence interval? Round your answer up to the next integer Answer(How to Enner) 1 Point Keypad Tables! Prev Next

Explanation / Answer

Step 1: Find z a/2 by dividing the confidence interval by two, and looking that area up in the z-table:
.99/2 = 0.495. The closest z-score for 0.495 is 2.58.

Step 2: Multiply step 1 by the standard deviation.
2.58 * 3.7 = 9.546

Step 3: Divide Step 2 by the margin of error. Our margin of error (from the question), is 0.24.
9.546/0.24 = 39.775

Step 4: Square Step 3.
39.775*39.775= 1582.05

The reqired sample size is 1582.

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