We use the t distribution to construct a confidence interval for the population mean when the underlying population standard deviation is not known. Under the assumption that the population is normally distributed, find ta/2, df for the following scenarios. (You may find it useful to reference the ttable. Round your answers to 3 decimal places.) a. A 90% confidence level and a sample of 21 observations. b. A 95% confidence level and a sample of 21 observations. c. A 90% confidence level and a sample of 13 observations. d. A 95% confidence level and a sample of 13 observations. tal2,df

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