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