STUDENT CASE STUDY, PART TWO B: COHORT STUDY—SINGER-FREEMAN
MMR VACCINE AND AUTISM: SCIENTIFIC INQUIRY, ETHICS, AND EVIDENCEBASED PROBLEM SOLVING
CASE STUDY FOR THE AACU STIRS PROGRAM
Karen Singer-Freeman, Associate Professor of Psychology, Purchase College, State University
of New York
STUDENT CASE
Part Two B: Cohort Study
How Can We Study the Effects of Potentially Dangerous Treatments? Methods Used in the Study
of Vaccines and Autism
Cohort Study
When we examine naturally occurring differences in a population followed over time, we are
using a cohort study. We know that some parents decide not to vaccinate their children, while
most do. We could therefore compare autism rates between these two groups of children.
Cohort studies are useful because they allow us to study the effects of potentially dangerous
experiences without creating this danger in order to conduct the study. However, in a cohort
study we always must be careful about drawing causal conclusions. Because we did not
randomly assign subjects to groups there may be other differences between the groups that
cause the differences we believe to be resulting from our exposure (in this case, the MMR
vaccination).
For example, if we wished to know whether smoking causes heart disease it would be unethical
to randomly assign a group of people to start smoking (because we know that smoking causes
cancer). However, if we compare the heart health of an existing group of smokers to an existing
group of non-smokers there might be other differences between the groups that contribute to
heart health. For example, non-smokers might be more likely to exercise, get enough sleep, or
eat healthy food than smokers. Figure 9 summarizes the important features of Cohort Study
design.
Population of All Children
Vaccinated
Children
Measure Autism
Rates
Unvaccinated
Children
Measure Autism
Rates
Figure 1. Cohort Study Design to Examine the Association between the MMR Vaccine and
Autism.
STUDENT CASE STUDY, PART TWO B: COHORT STUDY—SINGER-FREEMAN
A Real Cohort Study
Madsen et al. (2002) completed a cohort study by examining the medical records of all Danish
children born between 1991 and 1998. Of the 537,303 children in this study, approximately 82
percent received the MMR vaccine. Vaccination was not associated with increased risk of
autism. Among the children who had autism, there was no association between the
development of their symptoms and their receipt of the vaccination.
Key Questions
28)
How might unvaccinated children (and families) differ from vaccinated children (and
families)? Describe at least two ways these children could differ.
29)
How could the differences you identified in your answer to question 28 influence the
prevalence of autism in children in each group?
One reason that people believed that the MMR vaccine caused autism is that this vaccine is
given shortly after the first birthday. Many children begin to display the first symptoms of
autism at around this time (Offit and Coffin 2003).
Human beings have many cognitive biases that affect the way they think about information.
One of these is the confirmation bias. The confirmation bias causes us to pay attention to
evidence that supports our beliefs and be less likely to notice information that disproves our
beliefs (Stanovich, West, and Toplak 2013). Let’s think about the way this might influence
parents’ perception of vaccines and autism.
Look at tables below, which summarize the data from the Madsen et al. 2002 study. If we
believe that the MMR vaccine causes autism, the confirmation bias might cause us to pay more
attention to children with autism who received the MMR vaccine and those without autism
who were unvaccinated (highlighted cells), ignoring the many disconfirming cases of individuals
who received the MMR vaccine and did not develop autism and those who developed autism
despite not being given the MMR vaccine. These numbers could reinforce our belief that the
MMR vaccine causes autism.
Tables Summarizing Data from Madsen et al. 2002
MMR Vaccine
Autism
No Autism
Vaccinated Children
A
269 Children
0.11%
B
STUDENT CASE STUDY, PART TWO B: COHORT STUDY—SINGER-FREEMAN
Unvaccinated Children
C
D
96,601 Children
99.95%
However, if the MMR vaccine actually caused autism it should influence the incidence of autism
so that the proportion of children with autism will be higher in the population of children who
received the MMR vaccine than in the population of children who did not receive the MMR
vaccine. An observational study that uses a cohort design (such as the one conducted by
Madsen et al. 2002) provides us with data for all four of the squares above (A, B, C, D). In this
way, a cohort study can help us to see whether children who are unvaccinated are more or less
likely to develop autism than those who are vaccinated. The data from Madsen’s study show
similarly small proportions of children developed autism in both groups. The percentage of
vaccinated children who developed autism (.11%) was not significantly different (p < .05) than
the percentage of unvaccinated children who developed autism (.05%).
MMR Vaccine
Autism
No
Autism
240,655 Vaccinated Children
A
B
269 Children 240,386
0.11%
Children
99.89%
96,648 Unvaccinated Children
C
D
47 Children
96,601
0.05%
Children
99.95%
STUDENT CASE STUDY, PART TWO B: COHORT STUDY—SINGER-FREEMAN
Key Questions
30)
Explain how a cohort study allows you to avoid being fooled by the confirmation bias.
31)
Explain why Madsen et al. (2002) did not conclude that the MMR vaccine causes autism
even though more vaccinated children developed autism than unvaccinated children.
32)
Propose another way to study the relation between the MMR vaccine and the
development of autism. Be sure to include the group or groups you would study, the
information you would gather, and how this study would add to our understanding of
vaccine safety.
Once your group is done with these questions, please see your instructor for guidance on next
steps.