**Student’s t-Distribution**

**Level of Significance for One-Tailed Test**

**df** **0.100** **0.050** **0.025** **0.01** **0.005** **0.0005**

**Level of Significance for Two-Tailed Test**

**df** **0.20** **0.10** **0.05** **0.02** **0.01** **0.001**

**30** 1.310 1.697 2.042 2.457 2.750 3.646

**40** 1.303 1.684 2.021 2.423 2.704 3.551

**60** 1.296 1.671 2.000 2.390 2.660 3.460

**120** 1.289 1.658 1.980 2.358 2.617 3.373

From a sample of 41 orders for an on-line bookseller, the average order size is $75, and the sample standard deviation is $18. Assume the distribution of orders is normal. For which interval can one be exactly 90% confident that the population mean is contained in that interval?

**How am I supposed to know to use the 0.05 aligned with the n-1 df for the level of sig. from the 1 tailed 0.05, rather than the 2 tailed table’s 0.05?**