What critical value of t to the th power times should be used for a 99 point 5 percent confidence interval for the population mean based upon a random sample of 18 observations? Use the t -table.

A 99.5% confidence interval leaves 0.5% outside the interval, or 0.25% in each tail. With 18 observations, the degrees of freedom are 18 − 1 = 17. The t-table value for 17 degrees of freedom and an upper-tail area of 0.0025 is 3.222, so the critical value is 3.222.

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