Controlling Extraneous Variables

Controlling Extraneous Variables

Chapter Objectives • Learn to control aspects of the physical environment • Understand demand characteristics and experimenter bias and how to control their effects • Learn how an experimenter’s personality can influence experiments • 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

Balancing • Distributing the effects of an extraneous variables across the different treatment conditions. • Balanced (assign subjects randomly)

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

Controlling Extraneous Variables