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. Be sure that you understand when to use the t test for dependent means (that was discussed in Chapter 9) and when to use the t test for independent means.
• The t test for dependent means is used to compare two groups of scores that come from the same group of people (such as the same people measured before and after a test preparation course).
• The t test for independent means is used to compare two groups of scores that come from two separate groups of people.
• You also know you have made a mistake if it does not come out closer to the estimate from the larger sample.
2. You know you have made a mistake in figuring S² if it does not come out between the two estimates from which it is calculated.
3. Remember that when figuring estimated variances, you divide by the number of degrees of freedom. But when figuring the variance of a distribution of means, which does not involve any additional estimation, you divide by the actual number in the sample.

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