experiments and quasi experiments n.
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Experiments and Quasi-Experiments. (significance of group differences). Overview. Up to this point we have been discussing the relationships amongst variables where the same subject answers multiple questions

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experiments and quasi experiments

Experimentsand Quasi-Experiments

(significance of group differences)

overview
Overview
  • Up to this point we have been discussing the relationships amongst variables where the same subject answers multiple questions
      • e.g., what is the relationship between height and weight, what predicts bar passage, etc.
  • We are now transitioning to a new topic of discussing group differences where different subjects are in different conditions
      • e.g., who is happier, males or females?
overview1
Overview
  • relationships amongst variables
      • regression lines
      • only tests “CORRELATION”
  • group differences
      • mean (average) of each group
      • can prove “CAUSATION”
  • The major advantage of “group differences” is proving causation
correlation v causation
Correlation v. Causation

Depressed Mood

Cause?

Impaired Sleep

Depressed Mood

Impaired Sleep

Cause?

Depressed Mood

Impaired Sleep

Cause?

Cause?

Family Conflict

correlation v causation1
Correlation v. Causation
  • Finding: Women who have a baby after age 40 are more likely to live page 100.
  • Finding: The greater the quantity of ice cream sold, the greater the number of murders.
  • Finding: The greater the number of Churches, the greater the amount of crime.
  • Finding: The more a person weighs, the larger his/her vocabulary.
experiments
Experiments

(1) random assignment of Ss

(2) to two or more conditions

(3) which differ in terms of (only) IVs

1 random assignment
(1) Random Assignment
  • What is random assignment?
      • every subject has an equal chance of being assigned to different conditions
  • Why do random assignment?
      • purpose is to prevent systematic and non-treatment differences among subjects in each condition
2 two or more conditions
(2) Two or more conditions
  • Two levels
    • “yes versus no” (manipulate happiness versus no emotion)
    • “high versus low” (manipulate high happiness versus low happiness)
    • “positive v. negative” (manipulate happiness versus sadness)
2 two or more conditions1
(2) Two or more conditions
  • Three+ levels
    • Allows you to see direction of the effect
    • Allows you to see shape of relationships
3 which differ in terms of only ivs
(3) Which differ in terms of (only) IVs
  • In experiments, you manipulate variables.
  • By only manipulating the IV, and keeping all other factors constant (via random assignment), then any change in the DV is due to the IV
  • Thus, you can prove the IV CAUSED the DV
comparing correlation designs and causation designs
Comparing Correlation designs and Causation designs
  • Does watching violent TV make children aggressive?
    • How would you conduct a correlational study testing this research question?
comparing correlation designs and causation designs1
Comparing Correlation designs and Causation designs
  • Does watching violent TV make children aggressive?
    • How would you conduct an experimental study testing this research questions?
quasi experiments
Quasi-Experiments
  • Quasi-experiments:
      • Contains aspects of both experiments and non-experiments because deficient in at least one of the three aspects of experimental designs

Two most important are:

    • (1) Within-subjects = measuring/manipulating same subjects at two or more times.
    • (2) Mixed-designs = containing both between-subjects and within-subjects designs
slide15

Quasi-Experiments

When do I choose which type of design?

  • Choose experiments!
  • If practical issues prevent you from conducting experiment, then those same practical issues will dictates which quasi-experimental design you use.