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Quasi-Experimental Designs. Quasi-Experimental Designs. Intermediate between correlational study and true experiment. More than a relationship between variables. Low internal validity = cannot determine causality.

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quasi experimental designs2
Quasi-Experimental Designs
  • Intermediate between correlational study and true experiment.
    • More than a relationship between variables.
    • Low internal validity = cannot determine causality.
  • In true experiment, IV is manipulated and subjects are randomly assigned to conditions.
  • In quasi-experiments, IV is “manipulated”, but subjects are already part of a group based on pre-existing characteristics.
nonmanipulated iv
Nonmanipulated IV
  • IV occurs naturally
    • Participants are not randomly assigned to conditions.
  • Compares performance between 2 or more groups based on pre-existing characteristics.
    • Ex: gender; religion; age; smokers vs. nonsmokers; high, medium or low cholesterol levels.
    • Groups are not equivalent before treatment.
    • Low internal validity – we cannot conclude causality
  • Nonmanipulated independent variable and measure a particular dependent variable.
control group nonequivalent group
Control group & Nonequivalent group
  • True experimental designs have an experimental group (treatment) and a control group (no treatment).
    • Participants are randomly assigned to either condition.
  • Quasi-experimental designs do not have a control group because there is no random assignment of participants to the conditions.
    • The nonequivalent group serves as the comparison to the treatment group
typical quasi experimental design
Typical quasi-experimental design
  • Select 2 groups based on pre-existing characteristics.
  • Divide each group in half: half of the participants in each group get the treatment and half do not.
  • Compare performance with and without IV within each group and across groups.
  • Disadvantage
    • Pre-existing differences can confound results.
slide6

Nonequivalent group design

Age

Males Females

Caffeine

Yes NO

DV: # of anagrams solved

slide7

Nonequivalent group design

Age

Young Old

Memory Test

RecallRecognition

DV: % of words remembered

single case experimental designs9
Single case experimental designs
  • Involves the study of only 1 participant (single case designs) or 2 or 3 participants (small- n designs)
  • Often used in clinical settings.
  • Do not allow for generalization.
  • Allow for replications with different IV on the same participant or small-n designs.
  • Do not compare means nor run statistical analyses.
    • Assess how performance changes from one condition to another by graphing it.
baseline measurement
Baseline measurement
  • A measurement of behavior made under normal conditions (e.g., no IV is present); a control condition.
  • Serves to compare the behavior as affected by the IV.
  • Collect enough measures to achieve a stable pattern.
representative single case experimental designs
Reversal Designs

IV is introduced and removed one or more times.

1) A-B design

- simplest of all designs

- measure baseline behavior, apply treatment and compare behavior after treatment to baseline.

- does not allow to establish cause-effect

Representative Single-Case Experimental Designs
slide12

A-B design

treatment

Behavior during/ after treatment

Behavior at Baseline

a b a design
A-B-A design
  • Baseline measurement
  • Apply treatment
  • Measure change in behavior (posttest 1)
  • Remove treatment
  • Behavior “should” go back to baseline

(final assessment)

slide14

A-B-A design

treatment

Behavior with treatment

Behavior at Baseline

Remove treatment

Behavior back to Baseline

a b a b design
A-B-A-B design
  • Baseline measurement
  • Apply treatment
  • Measure change in behavior (posttest 1)
  • Remove treatment
  • Behavior “should” go back to baseline (assessment)
  • Apply treatment again
  • Measure change in behavior (posttest 2)
  • More ethical to end with treatment.
slide16

A-B-A-B design

Remove treatment

treatment

Behavior at Baseline

Behavior with treatment

treatment

Behavior with treatment

Behavior back to Baseline

multiple baseline designs
Multiple-Baseline Designs
  • Effects of IV are assessed across several participants, behaviors and situations.
  • Control for confounds by introducing treatment at different times for different participants, behaviors and situations.
multiple baseline designs19
Multiple-baseline designs
  • Multiple-baseline across participants
    • Determine who has most stable baseline and introduce treatment to that subject first.
  • Multiple-baseline across behaviors
    • Determine most stable behavior and start with treatment on that behavior and then start on 2nd behavior.
  • Multiple-baseline across situations
    • Determine when behavior is occurring and tackle one situation at a time.