C ONTROLLING E XTRANEOUS V ARIABLES

C HAPTER O BJECTIVES 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

E XTRANEOUS V ARIABLES Variables that can threaten an experiment internal validity Physical Social Personality Context

P HYSICAL V ARIABLE 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

E LIMINATION Simply, eliminating variables that can confound the experiment Noise? Use a soundproof room., hang a “Don’t Disturb” sign

C ONSTANCY OF C ONDITIONS 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.

B ALANCING When neither elimination nor constancy can be used. Subjects not available at the same time. Limited use of rooms. Confounded Experimental GroupControl Group Room 801; 9am, July 15, 2009Room 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

B ALANCING Distributing the effects of an extraneous variables across the different treatment conditions. Balanced (assign subjects randomly) Experimental GroupControl Group Room 801; 9am, July 15, 2009Room 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

S OCIAL V ARIABLES Qualities of the relationship between subjects and experiments Demand characteristic Experimenter bias

D EMAND CHARACTERISTICS A spects 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

S INGLE - BLIND E XPERIMENT 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

C OVER S TORIES 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

E XPERIMENTER B IAS 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

D OUBLE -B LIND E XPERIMENT The subjects do not know which treatment they are receiving, and the experimenter does not know either. Use of an independent rater

P ERSONALITY V ARIABLES 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

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

W HEN S UBJECTS S ELECT THE E XPERIMENT 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

W HEN E XPERIMENTER S ELECTS THE S UBJECTS 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