These study tips are designed to clarify key points and help you to avoid errors that students commonly make. Review the Tips for Success as you study each chapter and review them again after you have studied each chapter.

1. Note that to find the variance of a distribution of means, you always divide the population variance by the sample size. This is true whether the population’s variance is known or only estimated. It is only when making the estimate of the population variance that you divide by the sample size minus 1 (degrees of freedom). That is, the degrees of freedom are used only when estimating the variance of the population of individuals.
2. Be sure that you fully understand the difference between S² and S²_M.
• Although these terms look similar, they are quite different: S² is the estimated variance of the population of individuals, and S²_M is the estimated variance of the distribution of means (based on the estimated variance of the population of individuals, S²).
3. Step 2 of the hypothesis testing procedure is very long for the t test for dependent means and it is easy to lose track of the purpose of that step. As you will hopefully recall, Step 2 establishes the characteristics of the comparison distribution, which in this case is a distribution of means of difference scores: those characteristics include its mean (μ, which is assumed to equal 0), standard deviation (which is estimated as S_M), and its shape (a t distribution with degrees of freedom equal to the sample size minus 1).

Copyright © 1995-2008, Pearson Education, Inc., publishing as Pearson Prentice Hall Legal and Privacy Terms