An educational psychologist wishes to know the mean number of words a third grader can read per minute.
she wants to ensure that the estimate has an error of at most 0.29 words per minute. a previous study found that the mean was 25.7 words per minute.

assuming that the variance is 8.41, what is the minimum number of third graders that must be included in a sample to construct the 85% confidence interval?

208.

Step-by-step explanation:

The formula to find the minimum sample size is given by :-

                                   (1)

, where z* = critical z-value (two tailed).                                                                      

= Standard deviation ( from prior study ) and E = Margin of error.

As per given , we have

Margin of error : E= 0.29

Confidence level = 85%

Significance level =

Using z-table , the critical value (two -tailed)=

As per previous study , Variance =

Now, the required minimum sample size =   [Substitute the values in formula (1)]

 [ Round to the next integer]

Hence, the minimum number of third graders that must be included in a sample = 208.

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