9.4 Design of Experiments

Learning Objectives

• Practice accessing academic research online.
• Practice reading academic research critically.

Locate Article

This assignment will be based on a research article with the following citation:

A Case-Control Study Evaluating the Relationship Between Thimerosal-Containing
*Haemophilus influenzae* Type b Vaccine Administration and the Risk for a
Pervasive Developmental Disorder Diagnosis in the United States. (2015)
Geier et al., Biol Trace Elem Res 163:28.

Locate the article online, read through it, and answer the questions below.

9.4.1 Critical Reading

The study examines the relationship between exposure to thimerosal-containing vaccines and pervasive developmental disorder (PDD). PDD is a general group of disorders, including autism.

1. Is this an observational study or an experiment?
   • Observational study
   • Experiment
   • Neither
2. What type of study is this?
   • Randomized Control Trial
   • Sample survey
   • Case-control study
   • Soloman 4 group design
3. How were cases defined for this study?
   • The cases were subjects who took thimerosal-containing vaccine
   • There were two types of cases –subjects diagnosed with PDD and subjects diagnosed with febrile seizure
   • The cases were subjects diagnosed with PDD
   • The cases were subjects diagnosed with and without PDD
4. How were controls selected for this study?
   • There was a separate group of controls for each case group: non-PDD subjects, and non-febrile seizure subjects
   • The controls were those who received thimerosal-free vaccine
   • The controls were a random sample of persons in this age group
   • The controls were those who didn’t receive the thimerosal-containing vaccine
5. Why do the non-PDD controls have to be enrolled from birth for 7.22 years?
   • To ensure that subjects were old enough to understand the experiment
   • The mean age of vaccination is 7.22
   • The mean age of initial diagnosis of PDD is 7.22
   • To ensure that the controls are, in fact, not going to be diagnosed with PDD
6. How many subjects were in the study?
   • 534 children
   • 6,420 children
   • 26,166 children
   • 117,163 children
7. In the abstract, an odds ratio of 1.97 was reported. What is the correct interpretation of this finding?
   • Subjects diagnosed with PDD had almost twice the dose of thimerosal in their vaccine than subjects not diagnosed with PDD.
   • The odds of the higher level of thimerosal exposure was almost 2 times greater for subjects diagnosed with PDD than the PDD-free controls.
   • The probability of the higher level of thimerosal exposure was almost 2 times greater for subjects diagnosed with PDD than the PDD-free controls.
   • The odds of being diagnosed with PDD are almost twice as high as being diagnosed with febrile seizure.
8. The study found a statistically significant relationship between thimerosal dosage and subsequent risk of PDD diagnosis. What would be an accurate comment about this conclusion?
   • This study provides evidence of an association between thimerosal dosage and risk of PDD.
   • This study confirms a causal relationship between thimerosal dosage and risk of PDD.
   • This study shows that PDD is more likely to be treated with thimerosal.
   • This study demonstrates there is no relationship between thimerosal exposure and risk of PDD.
9. Why did the study evaluate the association between febrile seizure and thimerosal exposure?
   • To serve as a “control” outcome – it is generally accepted that there is no linkage between febrile seizure and thimerosal; if an association was found in this study it would raise doubt about the validity of any other associations between thimerosal and developmental disorders.
   • To determine if thimerosal is associated with febrile seizures.
   • To compare the rate of febrile seizures and PDD.
10. Based on both this paper and the in-class assignment paper, what did we learn about thimerosal?
    • One said there is no effect and the other said there is negative effect. They contradict each other.
    • Thimerosal, regardless of level of exposure, is harmful to children.
    • Higher thimerosal levels in vaccines are positively associated with PDD, and thimerosal-free vaccines work just as well.
    • Neither paper found that thimerosal had an effect on autism or developmental disabilities.