1 / 15

Dependent t-tests

1. Dependent t-tests. When the two samples are correlated (i.e. not independent). Dependent? What’s that?. Well, not independent…2 ways… Same individuals measured twice (known as repeated measures , or within subjects variables) Pre-test, post-test

melba
Download Presentation

Dependent t-tests

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 1 Dependent t-tests When the two samples are correlated (i.e. not independent)

  2. Dependent? What’s that? • Well, not independent…2 ways… • Same individuals measured twice (known as repeated measures, or within subjects variables) • Pre-test, post-test • Each person receiving both experimental conditions • Matched subjects • Form pairs based upon pairs’ similarity on a variable; then assign one of each pair to condition A, & one to condition B • Twins studies are an example of this (matched on genes, therefore - supposedly - matching on all sorts of other things) 1 2

  3. Standard deviation of the distn. • SEM of difference between dependent means 1 Key point: SEM is reduced in proportion with the correlation between the 2 sets of scores (in comparison with independent formula for SEM)

  4. So why use paired samples? • Because of that correlation • The larger the r, the larger the reduction in SEM, and the likelier it is you’ll get significant results • Wise use of dependent samples will normally increase power, increase effect size, increase likelihood of significant result 1

  5. Dependent t-test in SPSS Data format: Data from each sample must now be placed in separate columns. Note each person’s data (one pair of scores) fits on each row 2 1

  6. Dependent t-test in SPSS SPSS procedure: choose the appropriate command… 1

  7. Dependent t-test in SPSS Choose variables: slide the pair over from here… 1 Choose variables: to here And select ok

  8. Dependent t-test in SPSS Descriptives SPSS output r between samples (justification for choosing the test) 1 2 Significance level 3

  9. Note: what if we’d assumed independence? 1 Weird: now it’s significant…but I thought the dependent t-test was more powerful???

  10. Note: what if we’d assumed independence? 1 & r was negative, right? So that means the SE term grows rather than shrinks in the paired t-test – meaning less likelihood of significance But look – you subtract the product of r and the SEM. 2 3

  11. How dependent samples normally work… 1 • To prove the point… 2

  12. How dependent samples normally work… • To prove the point… 1

  13. How dependent samples normally work… • To prove the point… 1 2 4 3

  14. Finally, for the skeptics… 1 • Comparing same data via independent t-tests… 2 3 4

  15. Finally, for the skeptics… • Comparing same data via independent t-tests… 1 2

More Related