In order to construct a 90% confidence interval for the population mean when the population standard deviation is unknown and the sample size is 18, you should use the t-value indicated as: A) t_0.10,18 B) t_0.10,17 C) t_0.05,18 D) t_0.05,17 E) t_0.90,17

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When the population standard deviation is unknown and the sample size is 18, the degrees of freedom are n-1, which is 17. For a 90% confidence interval, you need the t-value for the 5% in each tail, which is t_0.05,17.

In Order to Construct a 90% Confidence Interval for the Population