quasi-experimental designs
DESCRIPTION
Quasi-Experimental Designs. Distinction is the degree of control over internal validity. True Experiment. Manipulation of a variable Control of threats posed by confounding variables NE and QE compare scores from diff groups or conditions but there is no manipulated variable. Quasi. - PowerPoint PPT PresentationTRANSCRIPT
Quasi-Experimental Designs
Distinction is the degree of control over internal validity
True Experiment
• Manipulation of a variable• Control of threats posed by confounding
variables• NE and QE compare scores from diff groups or
conditions but there is no manipulated variable
Quasi
• “having some but not all of the features”• Examples: lack of control or comparison
group, one group give treatment and then assessed
• No true IV• Participants already part of a group with
preexisting conditions
Nonmanipulated IV
• Participants not randomly assigned to conditions but come as members of a condition
• Subject (participant) variable– Gender– Age
Single-Group Posttest-Only Design
• Single group of participants given a treatment then tested
• No comparison group or previous measurements to compare
• Cannot be used to draw conclusions about how an experiences has affected P’s
Single-GroupPretest/Posttest Design
• Measurements taken before treatment and after treatment.
• Differences compared and any changes assumed to be treatment
• Problem is still no comparison group
Single-Group Time-Series Design
• Single group of participants measured repeatedly before and after a treatment.
• Allows for better baseline before treatment and continued measurement after treatment
Nonequivalent Group Designs
• Groups / conditions created by differences in individuals before experiment
• No control over assignment of individuals to groups
• Examples: IQ, race, gender
Differential Research Design
• NE research design because no attempt to control for threat of assignment bias
• Researcher interested in differences between groups on certain variable
• Ex post factolooks at differences after the fact
NE Control Group Posttest-Only Design
• Two nonequivalent groups (NE) given treatment and then posttest measure
• NE groups used in applied research settings in which goal is to evaluate treatment to a preexisting group of ind
• Second, control group, used as comparison
NE Control Group Pretest/Posttest Design
• At least two NE groups given a pretest, then treatment, then posttest
• Still no random assignment• Treatment only given to Experimental group• Compare between groups and within groups
Pretest-Posttest NE Control Group Design
• Phase 1– Observe (measure) both groups
• Phase 2– Administer treatment to Experimental group– No treatment to control Group
0 X 0 (Experimental group)0 0 (control group)
• Purpose of Phase 1 to determine if groups equal prior to Phase 2 treatment
• Quasi-Experimentalthreats minimized
Threats still possible
• Differential effects– Differences between groups may be result of
different histories effects
• Similar to influences such as maturation, instrumentation, testing effects etc
Caution!
• Remember the problem with threat to internal validity with these designs
• Assignment bias differences between groups could reflect individual differences
