Selecting and Recruiting Subjects
One Independent Variable: Two Group Designs
Two Independent Groups
Two Matched Groups
Multiple Groups
What is the purpose of an experimental design?
The design of an experiment details an
experimenter’s plan for testing a hypothesis.
The design is the experiment’s structure or
floor plan—not the experiment’s specific content.
We can use the same design to investigate
different hypotheses.
Introduction
What determines an experimental design?
The experimental design is largely determined
by the experimental hypothesis.
A researcher mainly selects an experimental
design on the basis of three factors:
1. the number of independent variables in the
hypothesis
Introduction
What determines an experimental design?
2. the number of treatment conditions needed
to fairly test the hypothesis
3. whether the same subjects are used in each
of the treatment conditions
Introduction
What constitutes a between-subjects design?
In a between-subjects design, a subject
participates in only one condition of the
experiment.
Introduction
What determines whether we can generalize our
findings?
The representativeness of our sample
determines whether we can generalize our
results to the entire population from which
the sample was drawn.
Random sampling increases an experiment’s
external validity.
Selecting and Recruiting Subjects
What is the minimum number of subjects for each
group?
You should have at 10-20 subjects in each
treatment condition to detect a strong
treatment effect.
Fewer subjects in each condition risks not
detecting the effect of the IV on the DV.
Selecting and Recruiting Subjects
What is effect size and why is it important?
Effect size is a statistical estimate of the size
or magnitude of a treatment effect.
The larger the effect size, the stronger the
relationship between the independent and
dependent variables, and the fewer subjects
needed to detect a treatment effect.
Selecting and Recruiting Subjects
Explain the relationship between effect size and
sample size.
Effect size determines the number of subjects
required to detect a treatment effect.
Researchers determine the number of subjects
required for an expected effect size using power
charts or programs that incorporate these charts.
Selecting and Recruiting Subjects
What is a two group design?
A two group design involves the creation
of two separate groups of subjects.
Two versions of the two group design are
the two independent groups design and two
matched groups design.
One Independent Variable: Two Group Designs
What is a two independent groups design?
A design where there is one IV with two levels
and subjects are randomly assigned to one of
the two conditions.
This design includes the Experimental GroupControl Group design and Two-Experimental
Groups design.
Two Independent Groups
Why do researchers use random assignment?
Random assignment involves assigning subjects
to conditions so that each subject has an
equal chance of participating in each condition.
We use random assignment to equally distribute
subject variables between the treatment groups
to prevent them from confounding an experiment.
Two Independent Groups
How do experimental and control conditions differ?
The experimental condition presents a value
of the independent variable.
The control condition presents a zero level of
the independent variable.
Two Independent Groups
Explain an experimental group-control group design.
This is a two independent groups design.
The experimental group receives a level
of the IV and the control group receives
the same procedures, but receives no
treatment.
Two Independent Groups
Describe a two experimental groups design.
In a two experimental groups design, we assign
subjects to one of two levels of the independent
variable.
This design is appropriate if there is one
independent variable with two levels and if we can
assume that randomization will control extraneous
variables.
Two Independent Groups
What limits the effectiveness of randomly assigning
subjects to different conditions?
Random assignment works poorly with 5-10
subjects per condition.
Since people often differ on many subject
variables that could potentially confound your
experiment, random assignment may not control
all of them.
Two Independent Groups
When would you use a two independent groups
design?
This design is appropriate if there is one
independent variable with two levels and if
we can assume that randomization will control
extraneous variables.
Two Independent Groups
What is a two matched groups design?
In a two matched groups design, we:
1. match participants on a subject variable
correlated with the DV, and
2. randomly assign them to one of two
treatment conditions
Two Matched Groups
What is the purpose of matching?
Matching is used to create groups that are
equivalent on potentially confounding subject
variables.
Successful matching prevents selection threat
from undermining internal validity.
Two Matched Groups
What must we measure to form matched groups?
We need to measure an extraneous variable,
strongly correlated with the dependent variable,
that could confound our results if not controlled.
Two Matched Groups
How would you match on IQ using precision
matching?
Form pairs of identical IQ scores (120 with 120).
Randomly assign members of each pair to one
of two treatment conditions.
Two Matched Groups
How would you match on IQ using range matching?
Form pairs of scores that fall within a specified
range like 5 points (115 with 120, 135 with 140).
Randomly assign members of each pair to one
of two treatment conditions.
Two Matched Groups
How would you match on IQ using rank-ordered
matching?
Rank all IQ scores from highest to lowest.
Form pairs of scores that fall in adjacent ranks
(1st score with 2nd score, 3rd with 4th).
Randomly assign members of each pair to one
of two treatment groups.
Two Matched Groups
When should you use a two matched groups
design?
You should use a two matched groups design
when there are two levels of an independent
variable and there is an extraneous variable
we can measure that could affect the dependent
variable.
Two Matched Groups
Differentiate between multiple groups and multiple
independent groups designs.
A multiple groups design is a between-subjects
design with more than two levels of an
independent variable.
In a multiple independent groups design, we
randomly assign subjects to one of the treatment
conditions.
Multiple Groups
What is block randomization and what does it
guarantee?
Block randomization is a process for randomly
assigning equal numbers of subjects to
conditions.
The experimenter creates random sequences
of each experimental condition and subjects are
randomly assigned to fill each treatment block.
Multiple Groups
How should a researcher choose the number of
treatments?
The hypothesis, prior research, pilot study
results, and practical limits can all help determine
the number of treatments.
A researcher needs to answer the question:
“What will I gain by adding these extra conditions
to the experiment?”
Multiple Groups
What are the practical limitations on the number of
treatments?
Practical limitations include:



available subjects
time
expense
Multiple Groups
What is a pilot study and how can it aid an
experiment?
A pilot study is a trial run of the experiment that
uses a few subjects.
A pilot study can help the experimenter refine the
procedure or determine whether the experiment
is promising.
Multiple Groups
What is a pilot study and how can it aid an
experiment?
A pilot study can reveal whether:




you have allocated sufficient time
your instructions are clear
your deception worked
you need additional treatment conditions
Multiple Groups