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Eighteen subjects were randomly selected and given proficiency tests. The mean of this group is 492.3, and the standard deviation is 37.6. Construct the 95% confidence interval for the population mean.

Answer :

Final answer:

The 95% confidence interval for the population mean based on a sample mean of 492.3 and a standard deviation of 37.6 with 18 subjects is approximately (472.62, 511.98).

Explanation:

To construct a 95% confidence interval for the population mean when the sample mean is 492.3 and the standard deviation is 37.6 with a sample size of 18, you would typically use the t-distribution because the sample size is small (n < 30) and the population standard deviation is unknown.

Looking up the t-value with 17 degrees of freedom (n-1) for a 95% confidence interval, we get a value of approximately 2.110. The formula for the confidence interval is:

CI = ± t * (s / √{n})

Where CI is the confidence interval, t is the t-value for the desired confidence level, s is the sample standard deviation, and n is the sample size. Plugging in the values:

CI = ± (2.110) * (37.6 / √{18})

We find the margin of error to be:

Margin of Error = 2.110 * (37.6 / √{18}) ≈ 19.68

Thus, the 95% confidence interval is:

(492.3 - 19.68, 492.3 + 19.68) ≈ (472.62, 511.98)

We estimate with 95 percent confidence that the true population mean for all such proficiency tests is between 472.62 and 511.98.

Ninety-five percent of all confidence intervals constructed in this way contain the true value of the population mean for this type of exam score.

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