chapter 10 designing quantitative studies

Chapter 10

Designing Quantitative Studies

The Counterfactual MethodThe Counterfactual Method

• The counterfactual is what would have happened to the same people simultaneously exposed and not exposed to the causal factor.

• Effect represents the difference between the two.

CausalityCausality

• The Counterfactual Method

• Criteria for Causality—Lazarsfeld (1955)

1. Temporal

2. Empirical relationship

3. Relationship cannot be explained as being caused by a third variable

Research Design Terminology in the Social Scientific and Medical Literature

Experiments or Randomized Controlled Trial (RCT) Properties

1. Manipulation

2. Control

3. Randomization

ManipulationManipulation

• Doing something to study participants

• Experimenter manipulates the independent variable by administering a treatment (intervention) to some subjects and withholding it from others, or by administering some other treatment

Control GroupControl Group

• Researchers can expose the control group to various conditions:

– no treatment

– alternative treatment

– placebo

– standard treatment

– different doses of the treatment

– wait-list

Randomization (Random Assignment, Random Allocation)

• Involves placing subjects into treatment conditions at random

• Approximates the ideal—but impossible—counterfactual of having the same people in multiple treatment groups simultaneously

• Basic randomization

Small Table of Random Digits Small Table of Random Digits

Example of Random Assignment Procedure

Breakdown of the Gender Composition of the Three Groups

Experimental DesignsExperimental Designs

•After-only (posttest-only) design

•Before-after (pretest-posttest) design

•Solomon four-group design

•Factorial design

•Randomized block design

•Crossover (repeated measures) design

Experimental Designs Experimental Designs

Symbolic Representation of a Pretest-Posttest Experimental Design

R O1 X O2

R O1 O2

R = Randomization

O = An observation or measurement

X = An intervention

Factorial DesignsFactorial Designs

• Two or more variables are manipulated simultaneously

• Test both main effects and interaction effects

• Randomized block design

• Crossover design

Example of a Factorial Design Example of a Factorial Design

Quasi-Experimental and Preexperimental DesignsQuasi-Experimental and Preexperimental Designs

Nonequivalent control group pretest-posttest design (quasi-experimental)

O1 X O2

Nonequivalent control group posttest-only design (preexperimental)

One group pretest-posttest design (preexperimental)O1 X O2

Quasi-Experimental DesignsQuasi-Experimental Designs

• Time series design

• Nonequivalent control group before-after

design

Time Series DesignTime Series Design

O1 O2 O3 O4 X O5 O6 O7 O8

Other Quasi-Experimental DesignsOther Quasi-Experimental Designs

• Regression discontinuity design

• Quasi-experimental dose-response analyses

• Quasi-experimental (nonrandomized) arms of a PRPP randomization design

Nonexperimental (or Observational) Research

• Descriptive research

• Correlational studies

Designs of Correlational StudiesDesigns of Correlational Studies

• Retrospective (case-control) design

• Prospective (cohort) designs

• Natural experiments

• Path analytic studies

Continuum of Designs for Inferring Causality

Strongest Weakest

True experiment Quasi-experiment Pre-experiment Path analytic Prospective Retrospective Descriptivecorrelational correlational

Descriptive StudiesDescriptive Studies

• Prevalence studies

• Incidence studies

chapter 10 designing quantitative studies

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{ chapter 4: designing studies 4.3 using studies wisely