experiments and quasi-experiments (significance of group differences)
Post on 17-Jan-2016
217 Views
Preview:
TRANSCRIPT
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
• 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?
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
Depressed Mood Impaired Sleep
Depressed Mood Impaired Sleep
Family Conflict
Depressed Mood Impaired Sleep
Cause?
Cause?
Cause?
Cause?
• 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.
Correlation v. Causation
Experiments
(1) random assignment of Ss
(2) to two or more conditions
(3) which differ in terms of (only) IVs
(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• 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 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
• 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
• Does watching violent TV make children aggressive?• How would you conduct a correlational study
testing this research question?
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:• 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
When do I choose which type of design?
(1) Choose experiments!
(2) If practical issues prevent you from conducting experiment, then those same practical issues will dictates which quasi-experimental design you use.
Quasi-Experiments
top related