Chapter 13
Research In Depth:
Longitudinal and Single-Case Studies
Longitudinal Research
• Longitudinal Research: A design in which an
investigator studies the same people or the same
population (but different individuals) over time,
sometimes across decades
• Cross-Sectional Research: A design in which an
investigator studies groups of different people
who vary on some characteristic (e.g., age) at the
same time.
– Cohort Effects: In longitudinal research, differences in
people resulting from characteristics of the era or
social environment in which they grew up rather than
to age effects specifically. This is a potential problem
in cross-sectional research.
Longitudinal Research
Common Themes in Differences in Participants in
Longitudinal Research
Genetic differences that underlie behavior. Genetics may
underlie 25% of the variability in cognitive abilities, but
much less for personality characteristics (Schaie, 2000)
Environmental differences that affect behavior. Changes in
the social context, including cohort effects, may change
behavior.
Interaction between genes and environment. Genetic
effects may unfold because of environmental influences
such that genetically-influenced behaviors will differ
greatly in different environments.
Varieties of Longitudinal Research
• Trend Studies: Longitudinal research in which
an investigator samples randomly from a
population over time, with different
individuals constituting each sample
The Issue
• How widespread is youth suicide?
• It is a significant issues, particularly in some
ethnic groups.
Example: Trends Over Time
Question:
• How many students have suicide ideation in
the state of Vermont? What is the pattern
over time.
Method:
In a trend study, Vermont studied middle- and
high-school students regarding their thoughts
of suicide since 1995.
Example: Trends Over Time
• Results: Over 20% of students claimed to have
made a plan for suicide in 1995, but the
number declined over the next decade to just
over 10%. There was also a decline in the
number of students who required medical
treatment.
Example: Trends Over Time
Results of Vermont trend study on suicidal ideation
Example: Trends Over Time
Conclusion
• A troublesome number of students think of
suicide, although the percentage declined
between 1995 and 2005.
• It would be wise for the state to think about
interventions to prevent suicides.
Varieties of Longitudinal Research
Cohort Study:
• A type of longitudinal research in which an
investigator randomly samples from a
population selected on the basis of specific
characteristics.
– The Nurses Health Study began in 1976 as a
simple questionnaire study.
– The NHS now involves about 122,000 nurses and
collects data on lifestyle and diseases, and it
includes samples like blood and tumor tissues.
Varieties of Longitudinal Research
Cohort-Sequential Samples
• A type of longitudinal research in which an
investigator repeated measures a cohort
group (e.g., people 60 years of age) over time,
adding a new cohort (e.g., new 60-year olds)
in each wave in order to differentiate age
effects and cohort effects.
Varieties of Longitudinal Research
• Question: When children begin using alcohol,
does their consumption increase over time?
• Method: In a cohort-sequential design,
investigate the alcohol intake of different age
groups, following a group of 6th grade, 7th
grade, and 8th grade children over three years.
Varieties of Longitudinal Research
• Result: Among children using alcohol, as they
get older, their alcohol use increases in similar
ways.
• Conclusion: When children use alcohol early
(e.g., in the sixth grade), their consumption
increases in consistent ways.
Varieties of Longitudinal Research
Results of cohort-sequential study on alcohol use in children
Source: Bray, J. H., Adams, G. J., Getz, J. G., &Baer, P. E. (2001). Developmental, family, and ethnic influences on adolescent
alcohol usage: A growth curve approach. Journal of Family Psychology, 15 301-314. Copyright © 2001 by the American
Psychological Association. Adapted with permission.
Varieties of Longitudinal Research
Panel Studies
• A type of longitudinal research in which an
investigator studies the same individuals over
time.
– The most famous panel study is Lewis Terman’s
study of gifted children that began in the 1920s
and continues today, although the few people still
alive are quite old.
– A more typical example involved a study of
bullying that lasted one year, with data collected
in three waves (Bond et al., 2001)
Controversy: Student Achievement
• The National Assessment of Educational
Progress (NAEP) has investigated student
performance in different subjects for over
three decades
• Examination of reading, math, and science
scores, results suggest that compared to the
1970s, today’s students score comparably or
better much of the time, although scores have
fluctuated over the years.
NAEP Results
Controversy: Student Achievement
• In math and reading, students scored at least
as well in 2008 as they had in 2004 or in 1971
when the measurement began.
Issues in Longitudinal Research
Longitudinal Approaches
Type of Study
Comment
Retrospective Study--An
approach to studying change
over time that relies on
people’s memory and
recollections of the past.
This may be the only viable
approach to studying the
effects of a traumatic event
that occurred in the past.
Prospective Study–An
approach to studying change
over time that identifies
research participants at the
beginning of the project who
are followed throughout the
course of research.
This approach is viable when
a researcher can begin a study
at some critical time and is
able to follow people forward
in time from the event.
Issues in Longitudinal Research
Attrition
• The loss of participants in longitudinal
research due to death, disappearance, loss of
interest, etc.
– Attrition is one of the most serious
methodological problems associated with
longitudinal research.
– Those who drop out might differ in important
ways from those who remain, so conclusions
based on studies with significant attrition can be
suspect.
Issues in Longitudinal Research
• Attrition can make conclusions ambiguous
– Bond et al. (2001) studied bullying and found that
(a) boys had a greater attrition rate than girls and
(b) boys showed a lower incidence of depression
associated with bullying.
– Could it be that boys who were depressed
dropped out of the study? Or could it be that boys
do not get as depressed due to bullying as girls
do?
– Because of attrition, it is not clear what is
happening with boys who are bullied.
Issues in Longitudinal Research
Sometimes attrition may not make a difference
– LaGreca et al. (1996) studied students who had
suffered greatly due to Hurricane Andrew.
– The attrition rate was 22% over ten months, which is a
typical rate for this kind of study.
– The researchers compared those who left the study
with those who remained. There were no differences
with respect to grade, gender, ethnicity, or initial
symptoms of PTSD.
– The researchers concluded, based on the participants
who remained in the study, that the data are likely to
be valid and representative of the population.
Issues in Longitudinal Research
Reasons for attrition in the Hurricane Andrew study of PTSD.
Issues in Longitudinal Research
Steps Taken to Reduce Attrition in a Longitudinal Study of Heavy Drinkers
Preparation
Ensure accuracy in verifying dates of birth and middle names to make later tracking
easier
Identify a contact person who lives at a different address
Maintain contact throughout the project with such mailings as birthday cards or regular
newsletters
Persistence
Begin a trace of the person as soon as contact is lost
Make multiple phone calls to set up appointments for interviews
Show willingness to conduct interviews at times and locations convenient for the
respondent
Provide incentives to offset the inconvenience of participation
Wutzke et al. (2000) reported that they were able to maintain contact with about
three quarters of their sample of heavy drinkers over 10 years, a remarkable
statistic for their population, which is notable for very high levels of attrition.
Single-Subject Experimentation
Experimental Analysis of Behavior
• An approach typically involving one or only a
few individuals
– Measurements tend to be highly objective and
quantitative
Methods of Single-Case Designs
Types of Withdrawal Studies Involving Presentation, Then
Withdrawal, of a Treatment
Note:
A = No treatment (or withdrawal of treatment)
B = Application of treatment
ABA Design–Type of design in which researchers get a baseline
measurement of behavior (A), apply some treatment (B), then
withdraw it (A).
ABAB Design–Type of design in which researchers get a
baseline measurement of behavior (A), apply a treatment (B),
withdraw the treatment (A), then re-apply the treatment (B)
Multiple Baseline Design–Type of design that studies multiple
behaviors that may change across baseline, treatment, and
withdrawal phases
Methods of Single-Case Designs
Single-Subject Randomized Controlled Trials
(RCT)
• A research design involving the study of a
single person over multiple trials, with trials
involving application of the treatment and
trials with no treatment occurring in random
order.
– Also called an N of 1 randomized clinical trial
• This approach is often used in applied, clinical
settings.
Methods of Single-Case Designs
Necessary Criteria for Appropriate Use of Single-Case
Designs
The behavior or condition being studied must be chronic
and stable.
The treatment (e.g., a drug) should have a rapid effect
and rapid cessation of effect when withdrawn.
There needs to be a clear and objective outcome that can
be measured reliably (Cook, 1996).
Example of Single-Case Design
• Question: Will application of drugs help
restore concentration in a brain-injured
patient?
• Method: In an N of 1 RCT, administer a
placebo, lorazepam, or dextroamphetamine
sulfate on randomly selected days. [Placebo
on day 2, lorazepam on days 1 and 4, and
dextroamphetamine sulfate on days 3 and 5
with a day in between to eliminate carryover
effects of the drugs]
Example of Single-Case Design
• Result: Performance was better with
dextroamphetamine sulfate than with the
other two treatments.
• Conclusion: One of the drugs outperformed
the other drug and the placebo.
Methods of Single-Case Designs
Strengths of Single-Participant Designs
These designs provide extensive, rich detail about the
behavior of the person being studied.
These designs are highly appropriate for studying rare
people or rare conditions.
Results of single-case designs are good for generating
further research hypotheses and experiments.
Results of single-case designs can provide useful plans for
treatment and therapy.
Methods of Single-Case Designs
Weaknesses of Single-Participant Designs
There are questions of external validity: Will the
results generalize to other people?
There are questions of internal validity: There
are concerns over conclusions of causation if
there is no manipulation of an independent
variable.
Methods of Single-Case Designs
Misunderstanding About Single-Case Research
Most psychological research involves groups, so psychologists are not
familiar with the strengths of single-case designs.
Researchers may confuse the objective, controlled, and quantitative
single-subject experiments with more qualitative and subjective case
studies.
Some researchers are concerned that single-case studies have low
internal and external validity, even though many of the same concerns
hold true for research using groups.
Some researchers claim that single-case research is too subjective, as
when investigators study internal, emotional states, even though the
same concerns would hold for group research.
Case Studies
• Case Study: A research design involving the indepth study of one or a few people,
traditionally with no manipulation of
variables.
– Case studies are descriptive in nature and
generally do not lead to valid causal conclusions.
– Case studies can be useful for preliminary
research of rare cases.
• When AIDS first appeared, there were few cases, so
researchers relied on case studies.
A Case Study: Tasting Pointed Chickens and
Seeing Colored Numbers
A Synesthete
• A man remarked that the chicken he was cooking
was not done because it was too round and did
not have enough points, that angel food cake has
a pink smell, and that quinine (a bitter liquid) felt
like polished wood.
A Case Study: Tasting Pointed Chickens and
Seeing Colored Numbers
– The man was a synesthete who experienced a
stimulus with multiple senses (synesthesia).
– Synesthetes are rare, so they can only be studied in
single-case designs and case studies.
– Cytowic (1993) studied this man for two years,
demonstrating that when experiencing synesthesia,
there was minimal blood flow to the cortex and
maximal blood flow to the limbic system.
– Because the extensive study involved a single
person, it is not clear that every synesthete’s brain
acts similarly.