Quasi-Experiments

Quasi-Experiment Involves the comparison of pre-defined groups

Non-Equivalent Group Design “ex post facto” – A basic between-subjects quasi-experiment Criteria – Ps are not randomly assigned – A quasi-independent variable (Snow/Cholera Example)

Pre-Defined Group 1 Pre-Defined Group 2 Measurement Measurement What makes this this a quasi-experiment?

Non-Equivalent Groups Design Threats To Internal Validity: Non-Equivalent Groups Design Selection Bias Selection Bias Pre-existing differences between comparison groups are present before treatment Assignment Bias Assignment Bias Occurs whenever the assignment procedure produces groups that have different characteristics

Pre- & Post- Test Non-Equivalent Groups Design – Pre-defined groups are observed before and after treatment Allows the researcher to address assignment bias/selection bias Allows the researcher to address assignment bias/selection bias

Pre-Defined Group 1 Pre-Defined Group 2 Post-Measure Post-Measure Pre-Measure Pre-Measure

Pre-/Post- Test Non-Equivalent Groups Design Threat To Internal Validity: Pre-/Post- Test Non-Equivalent Groups Design Differential History Effects Differential History Effects Groups differ because of a difference in their history

Nonequivalent Groups Time-Series Design – Involves a series of observations before and after treatment for both a treatment and control group

Pre-MeasurePre-MeasurePre-MeasurePost-MeasurePost-MeasurePost-Measure Pre-MeasurePre-MeasurePre-MeasurePost-MeasurePost-MeasurePost-Measure

Nonequivalent Groups Time-Series Design Threat To Internal Validity: Nonequivalent Groups Time-Series Design Regression to the Mean Regression to the Mean Individuals tend to regress toward mean

So, what’s the difference? Experiment IV DV Control – Experimenter Controlled Selection/ Assignment Quasi-Experiment Missing one and/or more