Chapter 10Experimental Research: One-Way Designs

Demonstration of Causality

• Association (Correlation)

• Temporal Priority

• Control of Common-Causal Variables

Association

• In looking for a cause and effect relationship between an independent variable and dependent variable, we must have an association.

Temporal Priority

• If event A occurs before event B, then A could be causing B.

• However, if event A occurs after event B, it cannot be causing that event.

• If children view a violent television show before they act aggressively, the viewing may have caused the aggressive behavior. But the viewing cannot have been the causal variable if it occurred only after the aggressive behavior.

Control of Common-Causal Variables

• Although Association & Temporal Priority are required for causal relationships, they alone are not sufficient.

• To make causal statements, also requires the ability to rule out the influence of Common-Causal variables that may have produced spurious (fraudulent) relationships between the independent and dependent variables.• Experimental Manipulations allow the researcher to rule out the

possibility that the relationship between the independent and dependent variables are spurious.

One-Way Experimental Designs

• One-Way experiments have one independent variable.

Levels

• There can be two or more levels for any independent variable

• t-tests may only have two levels• ANOVAs may have many levels

Equivalence

• In experimental designs, the influence of common-causal variables is eliminated (or controlled) through creation of equivalence among the participants in each of the experimental conditions before the manipulation occurs.

Equivalence

• Equivalence can be created either through using different but equivalent participants in each level of the experiment (between-subjects design) or through using the same people in each of the experimental conditions (within-subjects design, repeated-measures design).

Random Assignment

• For a true experiment to occur, in addition to manipulation of the IV, there must be random assignment of participants to control and experimental conditions. For a true experiment, we may state there is a cause and effect relationship between the IV and DV.

• If there is no random assignment, and there is manipulation of the IV, then a quasi-experiment is being performed. For a quasi-experiment, we may make only modest (limited) claims for a cause and effect relationship between the IV and DV.

Detecting Curvilinear Relationships

• The problem is that some relationships are curvilinear such that increases in the independent variable cause increases in the dependent variable at some points but cause decreases at other points.

• Anxiety & Performance

Within-Subjects Designs, Repeated Measures

• Dependent t-Tests

• Within-Subjects ANOVA

Advantages of Repeated-Measures Designs

• Increased Statistical Power

• Economy of Participants – In a between-subjects design with 20 participants in two levels requires 40 participants. In a within-subjects design with 20 participants in two levels requires only 20 participants.

Disadvantages of Repeated-Measures Designs

• These difficulties arise because the same individuals participate in more than one condition of the experiment and the dependent measure is assessed more than once.

• Carryover – Since different levels of the independent variable are manipulated more than once for each participant, there may be carryover effects from previous manipulations.

• Practice & Fatigue – Since each participant is measured multiple times on the DV (testing, typing, anything with effort), fatigue may occur and have an effect on scoring.

Question 1

• In what ways are experimental research designs preferable to correlational or descriptive designs? What are the limitations of experimental designs?

Question 2

• Differentiate between random sampling and random assignment. Which is the most important in descriptive survey research, and why? Which is the most important in experimental research, and why?

Question 3

• What is the purpose of random assignment to conditions?

Question 4

• Consider the circumstances under which a repeated-measures experimental research design, rather than a between-measures research design, might be more or less appropriate.