CONTROLLING EXTRANEOUS VARIABLES

CHAPTER OBJECTIVES

1. Learn to control aspects of the physical environment

2. Understand demand characteristics and experimenter bias and how to control their effects

3. Learn how an experimenter’s personality can influence experiments

4. Understand how to control for special problems created by the experimental context

EXTRANEOUS VARIABLES

Variables that can threaten an experiment internal validity

Physical Social Personality Context

PHYSICAL VARIABLE

Day of the week, testing room, noise, distractions

Experimental group and control group were tested on two separate occasions with grossly different physical conditions

Can be avoided by: Elimination Constancy of condition Balancing

ELIMINATION

Simply, eliminating variables that can confound the experiment

Noise? Use a soundproof room., hang a “Don’t Disturb”

sign

CONSTANCY OF CONDITIONS

If you cannot eliminate extraneous variable, try to keep all aspects of the treatment conditions as nearly similar as possible.

Same... Color of the wall, comfort level, lighting,

instructions, time, ventilation, etc.

BALANCING When neither elimination nor constancy can

be used. Subjects not available at the same time. Limited use of rooms.

Confounded

Experimental Group Control Group

Room 801; 9am, July 15, 2009 Room 804, 3pm; July 17

Participants:

E1, E2, E3, E4, E5, E6, E7, E8, E9, E10, E11, E12, E13, E14, E15, E16

Participants:

C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15, C16

BALANCING

Distributing the effects of an extraneous variables across the different treatment conditions.

Balanced (assign subjects randomly)

Experimental Group Control Group

Room 801; 9am, July 15, 2009 Room 804, 3pm; July 17

Participants:

E1, E4, E6, E7, E9, E10, E11, E13

C3, C5, C7, C8, C9, C10, C11, C15,

Participants:

C1, C2, C4, C6, C12, C13, C14, C16

E2, E3, E5, E8, E12, E14, E15, E16

SOCIAL VARIABLES

Qualities of the relationship between subjects and experiments Demand characteristic Experimenter bias

DEMAND CHARACTERISTICS

Aspects of the experimental situation that demand the people behave in a particular way.

Our behavior is shaped by what we think is expected in a given situation.

Research subjects want to be good subjects They might try to guess the hypothesis; may

set out to prove or disprove it. Can be controlled by:

Single-blind experiment Cover story

SINGLE-BLIND EXPERIMENT

An experiment in which subjects do not know which treatment they are getting.

Some information about the experiment may be revealed

Placebo effect Even so, there is still a possibility, though

slim, that the subjects will figure out the hypothesis

COVER STORIES

A plausible, false explanation for the procedures used in the study, in order to disguise the actual research hypothesis.

Deception is a departure form informed consent

Whenever possible, do not use one Debriefing is required

EXPERIMENTER BIAS

An experimenter may unknowingly give subjects cues that tell them how he would like then to respond.

Gestures, tone of voice, behavior of the experimenter can vary systematically across treatment conditions; errors in recoding data

What can be done: Follow written direction, time the experiment, be

consistent, minimize personal contact. Can be controlled by:

Double-blind Experiment

DOUBLE-BLIND EXPERIMENT

The subjects do not know which treatment they are receiving, and the experimenter does not know either. Use of an independent rater

PERSONALITY VARIABLES

Personal characteristics of the experimenter

Be pleasant, but remember that you can affect the outcome of your experiment

Maintain consistency in your interactions The more you vary behavior, the more you

are likely to produce variability in the responses of your subjects.

Minimize contact Adhere strictly to the experimental

procedures

CONTEXT VARIABLES

Come about from procedures created by the environment, or context of the research setting. Subject recruitment Selection Assignment procedures

WHEN SUBJECTS SELECTTHE EXPERIMENT

Am I free? What kind of experiment is it? Titles of experiments can bias the sample Try to keep the titles as neutral as possible

WHEN EXPERIMENTER SELECTS THE SUBJECTS

If subjects are not selected randomly, your sample will be biased

Best to use people you do not know Set procedure for randomly selecting people

you approach Keep demeanor consistent Design random assignment procedure