Experimental Design
Showing Cause & Effect
Relationships
Limitations of Experiments
• Often criticized for having little to do with actual
behavior because of strict laboratory conditions
• Not always Ethical to create “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.
– Problem is you can’t control variables in a Natural
Experiment.
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.
– Rather than trying to "prove" their hypothesis that something will
happen, social scientists actually try to disprove the null
hypothesis – that something will NOT happen
– We assume the null hypothesis is correct (that nothing is going to
happen) 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 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
Choosing Your Groups
minimizing confounding variables/individual differences
• 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 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
• Causes something to happen
• 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
• Find DV first by asking:
– “What is the researcher measuring or looking for in this study?”
• Next, find IV by asking:
– “What do the researchers hope will cause the DV in this study?”
• Verify with 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
• “Outside factors” that could have caused
your results.
• Need to be controlled/eliminated 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
(Participant/Researcher Bias)
• Any changes in an experiment’s results
due to the subject or researcher
anticipating certain outcomes to the
experiment
• Change in DV produced by subject’s
expectancy that change should happen
• Researcher favoring one group over another
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 (participant bias) - 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.
Eliminating Bias: 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.
Eliminating Bias:
Single Blind Procedure
• An experimental procedure where the research participants
are ignorant (blind) to the expected outcome of the
experiment
Eliminating Bias:
Double Blind Procedure
• Technique in which neither the experimenter nor participant
is aware of the group to which participant is assigned
Experiments:
Data Analysis
Are My Results Valid & Reliable?
• Validity – Does the experiment measure and
predict what it is supposed to?
• Reliable – If repeated, will we get similar results?
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?
3 Types of Experiments
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.