Experimental Design
Showing Cause & Effect
Relationships
Limitations of Experiments
• Often criticized for having little to do with actual
behavior because of strict laboratory conditions
• Ethical considerations in creating some more “real
life” situations
• Natural Experiments – Study natural occurring event
to observe and measure the effects of something you
could not create or ethically do in a lab.
Definitions
• Hypothesis—A testable prediction of the outcome of the
experiment or research
• Null Hypothesis - the statement that the independent
variable will have no effect on the dependent variable.
– More than trying to "prove" their hypothesis, social scientists
actually try in their research to disprove the null hypothesis,
– We assume the null hypothesis is correct until we can
encounter scientific evidence to reject it.
– Helps to avoid confirmation bias
• Variables—factors that change in ways that can be
observed, measured, and verified
• Operational definition—precise description of how the
variables will be measured
Operational Definitions
• How the researcher will define and
measure the key variables in the
experiment.
• In evaluating others’ research, first
determine if you agree with the
researchers’ operational definitions.
Experimental Design
• Randomly Select a Random Sample—every
member of the population being studied should have
an equal chance of being selected for the study
• Random assignment—every subject in the study
should have an equal chance of being placed in
either the experimental or control group
• Randomly select a random sample then randomly
assign that sample to the experimental and control
groups.
• Randomization helps avoid false results & bias &
accounts for individual differences in people.
Experimental Group
• The subjects in an experiment who
are exposed to the treatment
(independent variable)
• Also called the experimental
condition
• The group being studied and
compared to the control group
Control Group
• Are not exposed to the independent
variable
• Results are compared to those of the
experimental group
• Also called the control condition
Experimental Variables
• Independent variable (IV)
– the controlled factor in an experiment
– hypothesized to cause an effect on another
variable
• Dependent variable (DV)
– the measured facts
– hypothesized to be affected
Independent Variable
• The experimental variable which causes
something to happen
• The “cause variable”
• The variable manipulated by the
experimenter
• The variable which should change the
dependent variable
• variable is controlled by the experimenter
Dependent Variable
• The experimental variable which is
affected by the independent variable
• The “effect variable”
• The outcome of the experiment
• The variable being observed and
measured
Knowing the Difference
• Use an If/Then Statement
• If this (independent variable) THEN this happens
(dependent variable).
• If my subject drinks an energy drink (Ind.
Variable) THEN they should get a surge in energy
(Dep. Variable)
OR
• They are testing the effect of (IV) on (DV).
• Good Way to Remember: An IV in your arm
causes something to happen (DV)
Potential Problems
Experimental Flaws to Look Out For
Confounding Variables
• Variables, other than the independent
variable, which could inadvertently
influence the dependent variable
• These variables should be controlled for in
order to draw a true, cause-effect
relationship in the experiment.
• Many confounding variables can be
eliminated through random assignment.
Confounding Variables:
Environmental Differences
• Any differences in the experiment’s
conditions--between the experimental and
control groups
• Differences include temperature, lighting,
noise levels, distractions, etc.
• Ideally, there should be a minimum of
environmental differences between the two
groups.
Confounding Variables:
Expectation Effects
• Any changes in an experiment’s
results due to the subject anticipating
certain outcomes to the experiment
• Change in DV produced by subject’s
expectancy that change should happen
Sources of Bias
• Demand characteristics—subtle cues or signals by
the researcher that communicate type of
responses that is expected.
– Form of Researcher Bias
– Also helps to guard against the Clever Hans Effect
• Hawthorne Effect - refers to a change in behavior
of the subject because they have a great deal of
attention focused on them.
– Usually a spurt or elevation in performance or physical
phenomenon is measured.
Control of Bias
• Placebo control group—exposed to a fake
IV (placebo), the effects of which are
compared to group receiving the actual IV.
Placebo
• A non-active substance or condition administered
instead of a drug or active agent
• Given to the control group
• Reduces expectancy effects
• Ever get a boo boo and have your mom or dad to
kiss it and make it better?
• Doctors may use Placebos more than you think
(NBC Report on Placebo 2 min.)
• “Nocebo” – Patients when told a drug won’t work
can block it from working.
Blind procedure
• An experimental procedure where the
research participants are ignorant
(blind) to the expected outcome of the
experiment
• Sometimes called single blind
procedure
Double Blind Procedure
• Technique in which neither the
experimenter nor participant is aware of
the group to which participant is assigned
Experiments: Data
Analysis
Statistically Significant
• Possibility that the differences in
results between the experimental and
control groups could have occurred
by chance is no more than 5 percent
• Must be at least 95% certain the
differences between the groups is due
to the independent variable
Experiments:
Replication
Replication
• Repeating the experiment to
determine if similar results are found
• If so, the research is considered
reliable.
• Does Vitamin C really prevent colds?
Experimental Method
• Play “Water, Water Everywhere”
(12:20) Segment #2 from Scientific
American Frontiers: Video Collection
for Introductory Psychology (2nd
edition)
– Dousing Rods to find water
– An experiment is set up to see if this
psychic phenomenon is true.