Nonexperimental and 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

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 facto  looks at differences after the fact

NE Control Group Design Nonequivalent 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

Posttest-only nonequivalent control group design “static group comparison” One group of p’s given treatment and then measured after treatment (posttest). Scores then compared to control group X 0 (treatment group) 0 (control group)

Caution! Remember the problem with threat to internal validity with these designs Assignment bias  differences between groups could reflect individual differences

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-Experimental  threats 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

Pre-Post design One group of Ps measured before and after a treatment Goal is to evaluate influence of intervening treatment No control group Series of measurements over time on same Ps Caution: time-related threats