Experimental Design:
Introduction
Martin, Chapter 1
Psychology as a science

“Psychology is the Science of Mental Life,
both of its phenomena and their conditions.
The phenomena are such things as we call
feelings, desires, cognitions, reasonings,
decisions and the like; and, superficially
considered, their variety and complexity is
such as to leave a chaotic impression on the
observer.” William James (1890)
Methods for Investigating Hypotheses
(Establishing Cause and Effect Relationships)
Correlational research
 Quasi-experimental research
 Experimental research

Quantitative vs. Qualitative research
Kinds of Variables
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Independent Variables
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Dependent Variables
–
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what you hold constant
Random Variables
–

what you measure
Control Variables
–
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what you manipulate
what you allow to vary randomly
Confounding variable
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correlated with independent variable
Correlation
Take two measures from a sample; calculate
correlation coefficient
 Possible questions:

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–

is there a relationship between smoking and lung
cancer?
is there a relationship between anxiety and testtaking performance?
Correlation does NOT imply causation
Example of correlation
Experiments

Systematically vary variables of interest
–

e.g., giving different drugs
Critical concepts
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–
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Variable must be manipulated by experimenter
Random assignment of participants to
conditions
Avoid confounding variables
Example of experiment
Heart rate
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90
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60
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40
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10
0
No drug
Sugar pill
Condition
Cocaine
Quasi-experimental

Separate participants based on some
characteristic, e.g.:
–
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Possible questions
–

Gender, occupation, verbal ability (VSAT)
Do people with high verbal ability learn new
languages faster?
Accepted, but effects may be due to another
factor
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e.g., high-verbal people went to better high-school
How to select a kind of study
1) representativeness
–
does this seem like the real thing?
2) control
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can you manipulate something?
3) cost & effort
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is it worth it to do it the painful way
4) availability
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are the best kinds participants available?
Randomization
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Random Sampling
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Choose participants randomly from the entire
population
Allows generalization to population
Random Assignment
–
Methods for achieving random assignment
 Flip
coins; random numbers on arrival
 Assign conditions in blocks
Demo:
Can you pick a random number?
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