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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Experimentsand 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 Cause? Impaired Sleep Depressed Mood Impaired Sleep Cause? Depressed Mood Impaired Sleep Cause? Cause? Family Conflict
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 (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
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.