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QUASI-EXPERIMENTAL DESIGN

QUASI-EXPERIMENTAL DESIGN. Karen Sanders Lori Shough Anne Kerby Shelley Isai. Purpose & Goal. PURPOSE: to establish a cause-and-effect relationship between an independent variable and a dependent variable. However, assignment of subjects to treatment conditions is not at random.

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QUASI-EXPERIMENTAL DESIGN

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  1. QUASI-EXPERIMENTAL DESIGN Karen Sanders Lori Shough Anne Kerby Shelley Isai

  2. Purpose & Goal PURPOSE: to establish a cause-and-effect relationship between an independent variable and a dependent variable. However, assignment of subjects to treatment conditions is not at random. GOAL: to test cause and effect by observing how subjects react to phenomena.

  3. Three Types of Quasi-Experimental Design • Nonequivalent Comparison Group Designs • Interrupted Time-Series Design • Regression Discontinuity Design

  4. Non-Equivalent Comparison Group Design • Pretest/Posttest nonequivalent comparison group design: • Frequently used in education and social sciences • Comparison of two randomly assigned groups • Pre-test both groups • Treat one group • Post-test both groups • Selection bias is a threat to validity

  5. Static-group comparison • Comparison of two randomly assigned groups formed for purpose • Pre-test both groups • Treat one group • Post-test both groups • Selection bias is a threat to validity • Subjects are selected by either random or nonrandom methods and assigned to groups by nonrandom methods • No pretests given • The treatment conditions are implemented, and posttests are administered to both groups. • Disadvantages: • Questionable internal validity • Questionable external validity

  6. Interrupted Time-Series Design • In this type of research, a string of observations of the dependent variable is made (“pretests” - to establish an existing pattern) prior to the introduction of the treatment, after which another string of observations is made (“posttests”. If there is consistent and measurable change, it is considered reasonable to infer causality. • Particularly useful when all the individuals eligible for the treatment must be treated alike or when single individuals are treated. • Several threats to internal validity, including • History, Maturation, Testing, Instrumentation, and Regression.

  7. Regression Discontinuity Design • Assess the impact of treatments by measuring subjects in advance on some variable related to the outcome variable of the quasi experiment. • Knows precisely how the two groups differ on a relevant subject characteristic. • Subjects are assigned to the treatment because they need it or have earned it more than others. • The performance of the treated group is compared to the performance that one would predict if treatment had not been given. • The comparison group consists of the remaining individuals (those who did not need or earn the program).

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