Lab 21

Which Test?




Lab Questions

Use this file which represents the results of study on the effects of psychotherapy on aggression and empathy. 38 participants filled out a questionnaire measuring their aggressiveness and empathy before and after therapy. You’ll have to figure out whether to use a z-test or one of the 3 types of t-Tests. In at least one question, you'll need to create a difference score in the file.

How to create a difference score

Click Transform à Compute. Then name a new variable in the “Target Variable:” box. You can call it “AggressionChange” or anything you want. Then click on the “Aggression Scores After Treatment (aggression2)” variable and click the arrow button. In the “Numeric Expression:” box, you should now see “aggression2”. Now click the minus button (-). Click the “Aggression Scores Before Treatment (aggression1)” variable and click the arrow button. In the “Numeric Expression:” box, you should now see “aggression2-aggression1” just like in the picture below. Click OK to create the difference score.

 

The meaning of the difference score depends on the meaning of the variables and the direction of the subtraction.

  • In the case above, a negative score would be an improvement (if aggression2 – aggression1 is negative, the person became less aggressive) and a positive score would be a bad thing (if aggression2 – aggression1 is positive, the person became more aggressive).
  • If you wanted to make improvements be positive numbers, simply reverse the numbers in the difference score formula (COMPUTE AggressionImprovement = aggression1- aggression2 .).
  • Look at what happens if you create an empathy difference score, like this: COMPUTE EmpathyChange = empathy1 – empathy2 . A positive score would mean that someone was less empathic after treatment, a bad thing.
  • If you reversed the formula (COMPUTE EmpathyChange = empathy2 – empathy1 .), a positive score would be a good thing because it means that the person was more empathic after treatment.


Download the worksheet here to answer the questions.
Email it to your GA when you are finished.