
Lab #1

Intro to Experiments and Experimental Design
2

Experiment is a study in which a treatment is
introduced

The type of experiment (such as a true,
randomized experiment) depends on several
factors– the elements of design dictate the
type of experiment you have

Non-Experiments include responses from
natural groups (e.g., most surveys are nonexperiments)

http://imgs.xkcd.com/comics/the_data_so_far.png
Just because you manipulate something, it
does not make it an experiment.
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
Because we want to test specific,
directional hypotheses – not fishing
expeditions looking for relationships.

Because we want to isolate cause from
effect.

Because we want tight control of
conditions to rule out spurious
relationships.

Because we need carefully-constructed
designs that can be replicated to verify
findings.
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Tuskegee Syphilis
Study
Milgrim Obedience
Study
Stanford Prison
Experiment
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

Recruitment for
online surveys via
extrinsic and
intrinsic
motivation
Studying
development of
trust in different
types of social
interactions
No money
Gift card
$15
$60
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
(1) Independent Variables
must be manipulated
(usually by experimenter,
sometimes by context)

(2) Participants must be
assigned randomly to
various conditions or groups
Courtesy
http://psychology.ucdavis.edu/SommerB/
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
Independent variable manipulated by
experimenter:
 Whether there was any peer pressure to
give incorrect answer or not.

Random Assignment to Control group or
Treatment Group:
 In treatment group, confederates
answered incorrectly and the subject
answered last
 In control, no pressure to give incorrect
answer
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
Active independent variable(s):
 The I.V. is “given” to the participants,
usually for some specified time period. It
is often manipulated and controlled by the
investigator.

Attribute independent variable(s):
 A predictive, defining characteristic of
individuals. Cannot be manipulated.
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
Random Sample
 System for choosing
participants from a population
 Generally, larger sampling
population leads to better
generalizability.

Random Assignment
 Method for assigning
participants randomly to
experiment conditions
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
If we cannot randomly assign subjects to treatment and
control groups, then it is a quasi-experiment. The
independent variable is active.

If we cannot randomly assign subjects to groups
because the groups contain the attribute of interest, and
if we give all groups the same treatment, then it is an
associational non-experiment. The independent
variable is not active.

True experiments protect against both time and group
threats to internal validity by randomly assigning
subjects to treatment and control groups. The
treatment (independent variable) is active.
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Hypothesis  Random Assignment  Measure D.V.  Treatment  Measure D.V.
(pre-test)
(post-test)

Pre-test: allows us to check group equivalence before the
intervention X is introduced.

Experimental manipulation/Treatment: An independent
variable (X) that the experimenter manipulates.

Post-test: allows us to check group equivalence after
intervention X has been introduced.
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O
X
O
Pretest-Posttest Control
R
O
X
O
O
Post-only Control
R
O
O
R
X
O
X
O
(1)
O
(2)
O
(3)
O
(4)
Solomon 4-group
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
Question: Do individuals in the US and Japan
make different choices about ‘unique’ versus
‘less unique’ items when given a choice?
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
Independent Variables
 Cultural difference: Japanese students compared to US
students
 Pen layout (3 pens of one color, 1 pen of another color)
▪ Everyone received the same treatment

Assignment
 Subjects were not randomly assigned because they
already fell into one of the two societies.

Dependent Variable:
 Would they choose the ‘common’ pen or the ‘unique’ one?
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
Question: Do increased risk-taking behaviors
over time increase interpersonal trust?
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
Independent Variable
 Experiment Condition (2 conditions):
 Player Cannot control amount of money to
entrust to partner
 Player Can control amount to entrust to
partner

Assignment
 Random assignment of participants to one of the 2
conditions.

Dependent Variable
 Cooperation rate (i.e., whether coins were returned by
the partner or not)
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