Questions
 What is the relationship between ‘research
designs’ and ‘research strategies’?
 Which method of experiments, within subjects or
between subjects design is used more?
 In the staged emergencies in field studies, are the
people that are observed being deceived?
 Are simulations most commonly used to test social
norms?
 Has it ever been the case that a study is tested
with different designs? Is it effective to do this?
Questions
 How do researchers deal with threats to
internal validity, such as compensatory
equalization?
 Is it ethical to give a participant a placebo if
the drug being tested is needed by the
participant?
 How soon can we get our grade for our
presentations? Do you have a checklist for
all of the parts we need to include?
Experimental Designs: Withinsubjects design
Chapter 9
Dusana Rybarova
Psyc 290B
May 25 2006
Outline:
1. Introduction – Characteristics of withinsubjects designs
2. Advantages and disadvantages of withinsubjects designs
3. Dealing with time-related threats and order
effects
4. Applications and statistical analyses of
within-subjects designs
5. Matched-subjects designs
1. Introduction – Characteristics of
within-subjects designs
 a within subjects experimental design compares two or
more different treatment conditions (or compares
treatment and control) by observing or measuring the
same group of individuals in all of the treatment
conditions being compared
 a within-subjects design looks for differences between
treatment conditions within the same group of
participants
 a within subjects design is often called a repeatedmeasures design because the research study repeats
measurements of the same individuals under different
conditions
 it is used in experimental situations comparing different
treatment conditions and also to investigate changes
occurring over time
2. Advantages and disadvantages of
within-subjects designs
 Advantages of within-subjects designs
– it requires relatively few participants
– it essentially eliminated all of the problems
based on individual differences that are the
primary concern of a between-subjects designs
 a within-subjects design has no differences between
groups
 each individual serves as his or her own control or
baseline
2. Advantages and disadvantages of
within-subjects designs
 Disadvantages
– time-related problems
 participant attrition
 history
– any outside events that occur during the time that a withinsubjects experiment is being conducted and has an
influence on the participants’ scores
 maturation
– any physiological or psychological changes that occur in a
participant during the time a within-subjects experiment is
conducted and that can influence the participants’ scores
(e.g. young children)
2. Advantages and disadvantages of
within-subjects designs
 Disadvantages
– time-related problems (cont.)
 instrumentation
– refers to changes in the measurement instrument that occur
over time (e.g. observer changes)
 statistical regression
– or regression toward the mean is a mathematical
phenomenon in which extreme scores (high and low) on
one measurement tend to be less extreme on a second
measurement (especially a problem when participants are
selected for their extreme scores)
2. Advantages and disadvantages of
within-subjects designs
 Disadvantages
– order effects
 carryover effects
– changes in behavior or performance that are caused by
participation in an earlier treatment condition
– carryover effects exist whenever one treatment condition
produces a change in the participants that affects their
scores in subsequent treatment conditions (e.g. new skill
from treatment 1 can influence results in treatment 2)
 progressive error
– changes in participant's behavior or performance that are
related to experience over time in a research study but
not related to a specific treatment or treatments (e.g.
practice effects and fatigue)
3. Dealing with time-related threats
and order effects
 controlling time
– if the different treatment conditions are scheduled over a
period of weeks, the chances greatly increase that the
results will be influenced by some outside event
(history) or maturation or change in the measurement
instrument
 when a within-subjects design is not a good idea
– e.g. comparing two methods of teaching reading to firstgrade children (carryover effects)
3. Dealing with time-related threats
and order effects
 counterbalancing
– involves changing the order in which treatment
conditions are administered from one participant to
another
– the goal is to use every possible order of treatment
with an equal number of individuals participating in
each sequence
– the purpose of counterbalancing is to eliminate the
potential for confounding by disrupting any systematic
effects from factors related to time or the order of
treatments
– e.g. with two treatments one half of the participants
begins in treatment 1, then moves to treatment 2 and
the other half begins in treatment 2, then receives
treatment1
4. Applications and statistical analyses of
within-subjects designs
 two-treatment designs
– a repeated-measures t test or an analysis of variance
can be used to evaluate the statistical significance of
the mean difference
– if the data are measured on an ordinal scale, a
Wilcoxon test can be used to evaluate significant
differences
 multiple-treatment designs
– with too many treatment conditions, the distinction
between treatments may become too small to
generate significant differences in behavior
– statistical analysis – repeated-measures analysis of
variance to test for any significant differences among
the treatment means
5. Matched-subjects designs
 each individual in one group is matched with a
participant in each of the other groups
 the matching is done so that the matched
individuals are equivalent with respect to a
variable that the researcher considers to be
relevant to the study (e.g. IQ)
 maintains all the advantages of between-subjects
and within-subjects designs without the limitations
of either (e.g. eliminates individual differences,
time-related factors and order effects)