Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 1 Chapter Ten Experimental and Ex Post Facto Designs

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 2 Independent and Dependent Variables Variable: any quality or characteristic in a research investigation that has two or more values. Cause-and-Effect Relationship: the extent to which one variable (the cause) influences another variable (the effect). Independent Variable: a variable that the researcher studies as a possible cause of something else; the variable that the researcher directly manipulates. Dependent Variable: a variable that is potentially influenced by the independent variable; a variable that is influenced by and to some extent depends on the independent variable.

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 3 The Importance of Control Internal Validity: the extent to which the design of a research study and the data it yields allows the researcher to draw accurate conclusions about cause-and-effect and other relationships. Without internal validity in experimental designs, the results are not interpretable. Confounding Variables: account for differences in two or more groups that are not attributable to the particular treatment or intervention being studied.

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 4 Strategies for Controlling Confounding Variables 1.Keep some things constant. 2.Include a control group. 3.Randomly assign people to groups. 4.Assess equivalence before the treatment with one or more pretests. 5. Expose participants to all experimental conditions. 6. Statistically control for confounding variables.

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 5 Categories of Experimental Designs Pre-Experimental Designs True Experimental Designs Quasi-Experimental Designs Ex Post Facto Designs Factorial Designs

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 6 Pre-Experimental Designs Not possible to show cause-and-effect relationships because (a) the independent variable doesn’t vary or (b) experimental and control groups are not comprised of equivalent or randomly selected individuals. Pre-experimental designs include: -case study design -one group pre-test/post-test design -static group comparison design (cross-sectional study)

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 7 True Experimental Designs Compared to pre-experimental designs, experimental designs offer a great degree of control and greater internal validity. True experimental designs include: -pre-test/post-test control group design -Solomon four-group design -post-test only control group design

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 8 Quasi-Experimental Designs Randomness is not possible or practical; can’t control for all confounding variables. Quasi-experimental designs include: -time series design -equivalent time samples design -equivalent materials design -nonequivalent control group (comparison group) design -counterbalanced design

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 9 Ex Post Facto Designs The researcher identifies events that have already occurred or conditions that are already present and then collects data to investigate a possible relationship between these factors and subsequent characteristics or behaviors. - like correlational research, ex post facto research involves looking at existing circumstances; - like experimental research, ex post facto research has clearly identifiable independent and dependent variables; - unlike experimental research, ex post facto research involves no direct manipulation of the independent variable – the presumed “cause” has already occurred.

Practical Research: Planning and Design, Ninth Edition Paul D. Leedy and Jeanne Ellis Ormrod © 2010 Pearson Education, Inc. All rights reserved. 10 Factorial Designs Examination of the effects of two or more independent variables in a single study. Factorial experiments can be used: To find out whether specified factors influence response to a given treatment To see whether two factors interact or potentiate each other To optimise response in screening experiments To reduce the chance of missing an effect because the chosen material is insensitive.