Chapter 6
Conducting an Experiment: General
Principles
Choosing a Methodology: The
Practicalities of Research
• What is an experiment?
– The researcher creates equivalent groups through
random assignment
– There is at least one systematically manipulated
independent variable
– The researcher compares the groups to see if their
behavior differs on the dependent variable
Determining the Causes of Behavior
Requirements for Determining Causation
Covariance rule: The two variables have to vary together in a
predictable fashion.
• This characteristic is correlation, which is necessary, but not
sufficient, to identify causes
Temporal precedence rule: The causal variable has to precede
the effect
Internal validity rule: The causal variable must be the most
plausible cause, and other causal variables must be ruled out
Causal ambiguity: The situation resulting when
you cannot identify one and only one plausible
causal variable because there are competing
variables that could have a causal relation.
The Logic of Experimental
Manipulation
The simplest experimental design
Experimental
Group that
receives the
treatment
Control Group
that does not
receive the
treatment
The Logic of Experimental
Manipulation
• Experimental Group—The group or groups that
receive a treatment.
– The simplest experiment has one experimental group, but
some experiments have more than one.
• Control Group—The group that receives no
treatment
– The experimental group is compared with the control
group
• Placebo Group—A group similar to a Control Group
but that receives a fake treatment.
Ethics in Experimental Clinical
Research
• People come to clinicians for treatment.
• Is it ethical to conduct research by assigning
them to conditions that might not actually
help them?
• Sometimes new treatments are superior, but
sometimes they are not.
• We rely on researchers’ expertise and external
review by a board that reviews ethical issues
Controversy: Withholding Treatment in
Medical Research
• Many medical treatments have not actually
been studied in experimental research, so it
might be a good idea to see if the treatments
actually work.
• Experiments are the best way to see if
treatments work.
– Research has revealed that a number of medical
treatments work no better than placebo
treatment.
Lack of Control in Experimental Research
Lack of Control in Experimental Research
Extraneous Variables—A variable that the
investigator is not studying that may affect the
behaviors being studied
Confound—A variable not controlled by the
experimenter that has a systematic effect on at
least one group in the experiment
Example of an Extraneous Variable
• Researchers found that children who had
night lights as infants became nearsighted
(myopic) afterward.
• When children did not have night lights, they
were less likely to show nearsightedness.
• Could the presence of night lights have caused
the myopia?
Example of an Extraneous Variable
Incidence of Myopia and Normal Vision
Percentage of Nearsighted Patients
70
60
No Night Light
50
Night Light
40
30
20
10
0
Normal
Myopic
Vision Status
Source: Quinn, G. E., Shin, C. H., Maguire, M. G., & Stone, R. A. (1999). Myopia and ambient lighting at night. Nature, 399, 113-114.
© Macmillan Magazines Ltd.. Reprinted by permission.
Example of an Extraneous Variable
• Are there other variables that might be
related to nearsightedness?
– Nearsightedness is partially hereditary.
– Nearsighted parents may need night lights.
– Their children may become nearsighted not
because of night lights, but because of the genetic
inheritance from parents.
• Subsequent research showed that night lights
are not causally related to nearsightedness;
the relation is only correlational.
Controversy: Do Women Fear Success?
• Research showed that women generated
more negative stories to statements like
“Joanne found herself at the head of his
medical school class” than men did to
statements like “John found himself at the
head of his medical school class.”
• Do the negative stores reflect women’s fear of
being successful?
Controversy: Do Women Fear Success?
• The stories involved paired names
(John/Anne) in which the female name was
generally associated with negative
stereotypes.
– When the names were balanced to be equally
positive, the difference in negativity of the stories
tended to disappear.
– It took over two decades to discover this
experimental flaw.
• Small changes in experimental designs can
have big effects.
Experimenter Effects
• Experimenter Bias—The tendency of
researchers to inadvertently affect
participants’ behaviors, obscuring the effect of
the independent variable
Participant Effects
• Participants try to figure out what the study is about
and may try to “help out” the experimenter by acting
“the right way”, which is not good for the study.
• Cover story—The researcher can create a story that
disguises the purpose of the study.
• Blind study
– Single blind study—The participants do not know
what condition they are in.
– Double blind study—Neither experimenters nor
participants know what group the participants are
in.
Participant Effects
Types of Participant Effects
Hawthorne Effect—The tendency by participants
to act differently than normal because they know
they are being studied.
Demand Characteristics—The tendency by
participants to respond to what they think the
experimenter wants (or demands) from them.
Evaluation Apprehension—The tendency to feel
inadequate or to experience unease when being
observed.
Interaction Effects Between
Experimenters and Participants
Types of Interaction Effects Between
Experimenters and Participants
Biosocial effect—Change in participant behavior
based on characteristics like age, sex, or race of
the experimenter.
Psychosocial effect—Change in participant
behavior based on supposed attitudes or
personality of the experimenter.
Realism in Research
• Mundane Realism—The characteristic of a
research setting such that it resembles a
situation in everyday life, rather than an
artificial (e.g., laboratory) setting.
• Experimental Realism—The characteristic of
research such that participants experience the
psychological state that the experimenter
intends to produce.