Experimental Designs

Experiments are conducted to identify how independent variables influence some change in a dependent variable.

Researcher-Related Threats to Internal Validity Experimenter Effect Experimenter Effect Observer Bias Observer Bias Researcher Attribute Effect Researcher Attribute Effect

Participant-Related Threats to Internal Validity The Hawthorne Effect The Hawthorne Effect Testing Effect Testing Effect Maturation Maturation Experimental Mortality Experimental Mortality Selection Biases Selection Biases Intersubject Biases Intersubject Biases Compensatory Study Compensatory Study Demoralization Demoralization

Procedure-Related Threats to Internal Validity History History Instrumentation Instrumentation Treatment Confound Treatment Confound Statistical Regression Statistical Regression Compensation Compensation

Exercising Control Creating Equivalent Groups (Treatment & Control) Creating Equivalent Groups (Treatment & Control) Manipulating an Independent Variable Manipulating an Independent Variable Controlling for extraneous variables Controlling for extraneous variables

Types of Experimental Designs Pre-Experimental Design Pre-Experimental Design Quasi-Experimental Design Quasi-Experimental Design “True” or Classical Experimental Design “True” or Classical Experimental Design

Pre-Experiments Little control exercised by researcher Little control exercised by researcher Conditions are not randomly assigned Conditions are not randomly assigned Independent variable is either manipulated or observed Independent variable is either manipulated or observed Types of Designs: Types of Designs: –One-Shot Case Study –One-Group Pretest-Posttest Design –Static-Group Comparison

Quasi-Experiments Some Control exerted by researcher Some Control exerted by researcher Groups not randomly assigned -- assigned by pretest or natural categories – called “matching” Groups not randomly assigned -- assigned by pretest or natural categories – called “matching” Independent variable is often observed in its naturally occurring context Independent variable is often observed in its naturally occurring context Tend to be field experiments Tend to be field experiments Types of Designs: Types of Designs: –Time-Series Designs –Nonequivalent Control Group Design –Multiple Time-Series Design

True or Classical Experiments Most controlled design Most controlled design Must have random assignment to groups Must have random assignment to groups Laboratory experiment Laboratory experiment Independent variable is manipulated Independent variable is manipulated Double-Blind Experiment is when the participants and those who have contact with the participants are unaware of the group to which a participant is assigned. Double-Blind Experiment is when the participants and those who have contact with the participants are unaware of the group to which a participant is assigned. Manipulation checks are used to ensure the operationalization of the independent variable was manipulated as intended. Manipulation checks are used to ensure the operationalization of the independent variable was manipulated as intended.

Types of Classical Experiments Pretest – Posttest Control Group Design Pretest – Posttest Control Group Design Posttest – Only Control Group Design Posttest – Only Control Group Design Solomon Four – Group Design Solomon Four – Group Design

Factorial Designs Used when there is more than one independent variable Used when there is more than one independent variable Examines complex causal relationships Examines complex causal relationships –how each independent variable affects the dependent variable (main effects) and how the independent variables combined affect the dependent variable (interaction effects)

RQ: How does the sex of the speaker and immediacy influence perceived credibility? Female Speaker/High Immediacy Female Speaker/ Mod Immediacy Female Speaker/Low Immediacy Male Speaker/High Immediacy Male Speaker/Mod Immediacy Male Speaker/Low Immediacy 2 x 3 Factorial Design Male Speaker Female Speaker High Immediacy Moderate Immediacy Low Immediacy