Textbook formula clarification: We use Formula 8.2 (p. 225) in two situations: 1) When population standard deviation (sigma) is unknown and sample size is large (N is more than 100-120). However, on the left side you should have Z obtained instead of T obtained: Z (obtained) = (X - )/(s/(N-1)) 2) When population standard deviation (sigma) is unknown, and sample is small (N is less than 100 - 120 cases). T(obtained) = (X - )/(s/(N-1)) In the next chapter, SPSS will be running the tests for you. SPSS is really smart and uses only T-distribution because: 1) It is much better for small samples than Z-distribution. 2) For big samples (when N exceeds 100-120) there is no difference between T- and Z-distributions