Quasi-Experimental Strategy: Nonequivalent Group, Time-Series, and Developmental Designs

1. Quasi-Experimental Strategy: Nonequivalent Group, Time-Series, and Developmental Designs Chapter 9 George S. Robinson, Jr., Ph.D. Department of Psychology North Carolina A&T State University

2. Quasi-Experimental Research Strategy • Involves a comparison of groups or conditions • Uses a nonmanipulated variable to define the groups or conditions • The variable is usually a participant variable or time variable

3. Nonequivalent Group Design • Researcher can not control the assignment of the groups • Thus, the groups of participants are nonequivalent

4. Nonequivalent Group Design – cont. • Threats to internal validity for nonequivalent group design • Assignment bias • Participants in one group necessarily have different characteristics from the participants in the other group

5. Nonequivalent Group Design – cont. • Differential research design (non-experimental design) • No manipulation • Uses participant characteristics to assign groups • Attempts to compare pre-existing groups • (e.g., IQ scores from Blacks and Whites, or Leadership scores from men and women) • (ex post facto – “after the fact”)

6. Nonequivalent Group Design – cont • Posttest-only Nonequivalent Control Group (static group comparison) • Uses pre-existing groups • One group serves in the treatment condition • The other group serves as a control group (does not receive the treatment) • There is no random assignment to groups • X = treatment O = measurement • X O (treatment group) • O (nonequivalent control group)

7. Nonequivalent Group Design – cont • Pretest-Posttest Nonequivalent Control Group Design • Compares two nonequivalent groups • One group is measured twice, once before treatment, and once after treatment • The other group is measured at the same two times, but does not receive treatment • O X O • O O

8. Nonequivalent Group Design – cont • Threats to a Pretest-Posttest Nonequivalent Control Group Design • History effects • Instrumentation • Practice effects and fatigue • Maturation • Regression toward the mean • Threats from differential effects (differential history, differential instrumentation, differential practice/fatigue, differential maturation, differential regression)

9. Time-Series Design • Time-Series Design • A series of observations is made over time • One-group posttest design (pretest-posttest design) • One measurement before treatment, and one measurement after treatment for a single group • O X O

10. Time-Series Design – cont. • Time-Series Design • Series of observations before treatment, and a series of observations after the treatment • O O O X O O O • Interrupted Time-Series Design • A series of observations before an event occurs, and a series of observations after an event; the event is not a treatment or manipulated by the experimenter

11. Time-Series Design – cont. • Equivalent Time-Samples Design • A long series of observations in which a treatment is alternatively presented and withdrawn • O = observation, X = treatment, N = no treatment • O O O X O O O N O O O X O O O N O O O X

12. Developmental Research Strategy • Developmental research strategy • Used to examine changes in behavior related to age • Cross-Sectional Research Design • Uses different groups of individuals • Each group represents a different age • The groups are measured at one point in time and then compared • Longitudinal Research Design • Measuring a variable (commonly) in the same individuals over time

13. Terminology in Quasi-Experimental and Developmental Designs • Quasi-independent variable • Variable that is used to differentiate the groups of participants or the groups of scores • Dependent variable • Variable that is measured to obtain a score for each individual