Ch 8: Experimental Design

Ch 9: Conducting Experiments

Uses of Experimental Research

Test hypotheses derived from theories

Test the effectiveness of a treatment or program

Examine the causes of behavior

Conducting Experimental Research

Manipulate independent variable to see effect on dependent variable

Compare groups in terms of their scores on the dependent variable

All other variables kept constant through direct experimental control and/or randomization

Independent Variable

This variable is manipulated (controlled) by the experimenter and has at least two different levels (conditions)

1000 mg

0 mg=placebo

Manipulating the Independent Variable Straightforward manipulations

Amount of substance administered Written instructions Verbal material Visual material

Staged manipulations Often employ confederates Events are staged or manipulated to:

Create some psychological state Simulate some situation that occurs in the real world

Dependent Variable

This variable is measured by the experimenter and is used to determine the effect of the independent variable

no pain mild moderate severe

Measuring the Dependent Variable

Types of measures• Self-report measures• Behavioral measures• Physiological measures

Sensitivity of the Dependent Variable

The independent variable can appear to have no effect on the dependent variable when there is a• Ceiling effect—participants quickly reach

the maximum performance level• Floor effect—a task is so difficult that

hardly anyone can perform well

Posttest-only Design Obtain two equivalent groups of

participants (R=random assignment) Introduce the independent variable Measure the effect of the independent

variable on the dependent variable

Posttest-only Design Involves random

assignment of individuals to the different conditions of the independent variable

Random assignment A method for placing subjects in conditions prior

to implementing the independent variable Every individual has the same chance of being

placed in a given condition

Pretest-posttest design

Same as a posttest-only design but adds a pretest before the experimental manipulation

Allows the researcher to ascertain if the groups are equivalent at the beginning of the experiment

Example: Are kids healthieron some dimension (weight,stamina) after going throughan athletics program?

1. Assess equivalency with small sample size

2. Assess mortality (attrition or dropout factor)

Advantages of the pretest-posttest design

Mortality Mechanical subject loss: equipment

failure or experimenter error leads to loss

Selective subject loss: some characteristic of participant is responsible for loss

Disadvantages of the pretest-posttest design

• Time consuming• Awkward to administer• Sensitizes participants to what is being studied Demand characteristics:

Cues and information a

participant uses to guide

their behavior in a

psychological study

“Is research on cultural acceptance?”

Demand characteristics

o Possible solutions:• disguise pretest• embed the pretest in another measure (filler questions)• concealed observation

Placebo control group —used to assess whether participants’ expectancies contribute to the outcome of an experiment

Internal validity

Occurs when we are able to confidently state that the independent variable caused the differences we observe

Causal inferences can be made when internal validity is present

Confounding

This occurs when the variable of interest and a different potential independent variable are allowed to covary

Represents an alternative explanation for a study’s findings

Threatens internal validity

Other Threats to Internal Validity

Intact groups Extraneous variables Experimenter effects

Intact groups

This occurs when groups are formed prior to the start of an experiment

Selection differences: systematic ways in which people can differ

Selection differences

Characteristics of people that differ or vary:Physical characteristics: sex, raceSocial characteristics: ethnicity, religion,

marital statusPersonality characteristics: extraversion,

emotional stabilityMental health characteristics: depression,

anxiety

Extraneous variables Other practical considerations that

may confound an experiment

Experimenter effects Biases that occur when

experimenters’ expectancies regarding the outcome of the experiment influence their behavior toward participants in different conditions

Control by automating procedures as much as possible

Double-blind experiment A procedure in which

both the participants and the experimenters are unaware of which condition is being administered

Controls for both demand characteristics and experimenter effects

Experimental control techniques

Methods for ruling out other possible causes for the outcome

Assigning Participants to Experimental Conditions

Independent groups design• Participants randomly assigned to conditions• Participants are in only one group

Low-meaningful

High-meaningful

15 randomly assigned

participants

Another 15

randomly assigned

participants

Meaningfulness

Assigning Participants to Experimental Conditions

Repeated measures design• The same participants are in all of the groups

Low-meaningful

High-meaningful

15 participants The SAME 15

participants

Meaningfulness

Repeated Measures Design

Advantages Fewer participants Extremely sensitive to statistical differences (more

likely to detect an effect of the IV on the DV) Disadvantages

Order effects Practice effects Fatigue effects Contrast effects

Minimizing order effects Counterbalancing

1. Complete counterbalancing—all possible orders of presentation are included in the experiment

Matched pairs design

Ensures groups are equivalent on the matching variable prior to the IVMatch participants on a

particular characteristicAfter matching, randomly

assign to experimental conditions