Question

An educational psychologist wishes to know the mean number of words a third grader can read per minute. She wants to make an estimate at the 95% level of confidence. For a sample of 582 third graders, the mean words per minute read was 24.1. Assume a population standard deviation of 3.7. Construct the confidence interval for the mean number of words a third grader can read per minute.

Answer 1

The 95% confidence interval for the mean number of words a third grader can read per minute is (23.8, 24.4).

Explanation:

We have to find our
`\alpha` level, that is the subtraction of 1 by the confidence interval divided by 2. So:

`\alpha = (1 - 0.95)/(2) = 0.025`

Now, we have to find z in the Z-table as such z has a p-value of
`1 - \alpha`.

That is z with a p-value of
`1 - 0.025 = 0.975`, so Z = 1.96.

Now, find the margin of error M as such

`M = z(\sigma)/(√(n))`

In which
`\sigma` is the standard deviation of the population and n is the size of the sample.

`M = 1.96(3.7)/(√(582)) = 0.3`

The lower end of the interval is the sample mean subtracted by M. So it is 24.1 - 0.3 = 23.8

The upper end of the interval is the sample mean added to M. So it is 24.1 + 0.3 = 24.4.

The 95% confidence interval for the mean number of words a third grader can read per minute is (23.8, 24.4).