In addition to the question below, I had another question. I read when calculating the T critical value by hand, first you:

"Step 1:Subtract one from yoursample size.This is your df, ordegrees of freedom. For example, if the sample size is 9, then your df is 8 - 1 = 7."

If I had a sample size of 29, would by df be 29-1=28? I don't understand why the above example shows the sample size is 9, but then says to use 8. Wouldn't it just be 9-1=8?

Use the standard normal distribution of the t-distribution to construct....

Use the standard normal distribution or the t-distribution to construct a 90% condence interval for the population mean. Justify your decision. If neither distribution can be used, explain why. Interpret the results. In a random sample of 22 mortgage institutions, the mean interest rate was 3.61% and the standard deviation was 0.38%. Assume the interest rates are normally distributed. Which distribution should be used to construct the condence interval? 0 A. Use a t-distribution because the interest rates are normally distributed and o' is known. 0 B. Use a normal distribution because n