The Experiment Flashcards
Quasi-Experiment
Cannot randomly assign people to the levels of the IV.
•Cannot control something of interest.
Ex: Drug exposure on a fetus.
True Experiments
Can randomly assign individuals to levels of the IV. (Random Assignment: every participant has an equal chance for being in the levels of the experiment.)
•Control everything
Ex: Alcohol and reaction time.
Independent Variable
The one we believe is causing the change. (The group your in)
Levels of the Independent Varaible
The different values of an independent variable.
Types of Experiments
- Between Subjects Design
* Within Subjects Design
Between Subjects Design
Groups of participants only receive one level of the IV.
•Different groups receive the different levels of the IV.
EX: Group 1: Alcohol
EX: Group 2: No Alcohol
Within Subjects Design
(aka: Pretest-Posttest or Repeated Measures Design)
Subjects receive all the levels of the IV.
EX: Alcohol and reaction time
EX: No Alcohol and reaction time
Goal of the Experiment
Internal Validity
Internal Validity
The point of a true experiment.
•Ability to say that the manipulation of the IV caused the changes in the DV.
•Rule out extraneous (confounding) variables: something else that could be causing the changes noted.
Threats to Internal Validity in Between Subjects Designs (aka: Pretest-Posttest or Repeat Measures Design)
- Selection
- Selection by Maturation
- Mortality
Threats to Internal Validity in Between Subject Designs due to Selection
Where the groups are equivalent before the study began. Self-selection threat.
Ex: Age and Political Views: How people feel about a topic comparing older and younger people.
Threats to Internal Validity in Between Subjects Design due to Selection by Maturation
(Maturation: the individual is developing and changing.)
Asking whether your two groups would have naturally grown apart even without the treatment.
•Weren’t equal to begin with and they would have grown apart anyway.
Threats to Internal Validity in Between Subjects Designs due to Mortality
Dropout rates or death.
More people dropout of one group than another.
Controlling Threats to Internal Validity in Between Subjects Designs
- Random assignment
* Matching
Random Assignment
every person has an equal chance.
Matching
Make samples equivalent in some way on important variables.
EX: Income or Age
Threats to Internal Validity in Within Subjects Designs
- Maturation
- History
- Instrumentation
- Mortality
- Regression to the Mean
Threats to Internal Validity in Within Subjects Designs due to Maturation
Growing up or changing.
Threats to Internal Validity in Within Subjects Designs due to History
Could other events in the participants lives have caused the pretest-posttest difference.
Threats to Internal Validity in Within Subjects Designs due to Instrumentation
Was the same instrument used in the pretest as in the posttest.
Threats to Internal Validity in Within Subjects Designs due to Mortality
Die or Dropout. Start and end with the same amount of participants.
Threats to Internal Validity in Within Subjects Designs due to Regression to the Mean
Statistical phenomenon that extreme scores are unlikely to occur repeatedly.
Additional Threats to Internal Validity in Within Subjects Design due to Carryover Effects
The effect of one treatment persists to influence the participants response to the next treatment.
Additional Threats to Internal Validity in Within Subjects Design due to Practice Effects
Similar to Maturation = Growing up or changing.
Additional Threats to Internal Validity in Within Subjects Design due to Order Effects
The position of a treatment in a series determines in part the participants response.
EX: Brush teeth then drink orange juice versus drinking orange juice then brushing teeth.