What critical value of t* should be used for a 99.5% confidence interval for the population mean based upon a random sample of 18 observations?Find the t-table here.

For a confidence interval for a population mean with 18 observations, use 18 − 1 = 17 degrees of freedom. A 99.5% two-sided interval leaves 0.5% outside the interval, or 0.25% in each tail, so the critical value is the t value with 17 degrees of freedom and cumulative area 0.9975. That value is t* = 3.222, choice D.

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