Selecting a Study Design

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Selecting a Study Design
2
Goals

• Describe the cohort study design.
• Describe the case-control study design.
• Compare situations in which cohort and
  case-control study designs should be used.

3
About Analytic Studies

• We can use analytic studies to test hypotheses.
• We want to know
• Whether there is an association between
  hypothesized exposure and disease,
• How strong the association is,
• What proportion of cases are due to exposure, and
• Whether there is an increased risk of disease
  with increased exposure (a dose-response
  relationship).
• Two common types of analytic studies are cohort
  study and case-control study.

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What is a Cohort?

• A cohort is a group of people who have
  something in common.
• Can represent the source populationthe
  population from which cases of disease arise.
• Examples of cohorts
• All employees in an office building
• Everyone who attended a football game
• All the residents of a neighborhood

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Cohort Studies

• Tend to be retrospective (exposures in the past
  in relation to disease that has already
  happened).
• Occurrence of disease in exposed group compared
  to occurrence of disease in unexposed group
  risk ratio.
• Risk ratio tells whether disease is associated
  with exposure and strength of association.

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Identifying a Cohort

• To use a cohort study, you must identify every
  person in the cohort.
• Possible when the group is small and well defined
  (e.g., wedding reception, cruise ship, school,
  prison).
• Option to interview every member of the cohort or
  a sample of the cohort.

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Identifying a Cohort

• Sometimes it may be difficult to define a
  suitable cohort.
• Can you find every single person who ate at the
  Main Street Deli on January 10-20?
• How would you locate every person buying and/or
  eating contaminated lunch meat from a local
  supermarket chain?
• An alternative the case-control study.

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Case-Control Studies

• The most frequently used type of study in
  outbreaks.
• Can be quickly implemented.
• Can be used when cohort study might be large and
  time-consuming.
• Identify people with disease (case-patients) and
  people without disease (controls), then ask
  everyone about past exposures.
• You already know who is sick through doctor
  diagnosis, lab culture, or health department.

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Case-Control Studies

• Calculate odds ratio to measure strength of
  association between illness and exposure.
• Compare odds of exposure among case-patients to
  odds of exposure among controls.
• Cannot calculate risk ratio in case-control study.

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Selecting Cases and Controls

• Defining the source population may help narrow
  down potential controls.
• Do the cases live in the same city or attend the
  same event?
• Are they of a particular race or ethnicity?
• Understanding where cases came from will help
  select your controls.
• Controls are a sample of people from the source
  population.

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Selecting Cases and Controls

• Example Outbreak of gastrointestinal illness
  linked to eating at the Main Street Deli during
  January 10-20.
• Cases recruited from people who ate at the Deli
  and experienced vomiting. Controls recruited
  from people who ate at the Deli but did not
  experience vomiting.
• All cases recruited into study only a portion of
  healthy controls contacted because could not
  identify every person who ate at the restaurant
  during these 10 days.
• Want to know what case-patients and controls ate.
  
• Controls selected from customers who ate at the
  Deli during the time period of interest.

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Case-Control or Cohort Which one is right?

• The choice depends on the situation.
• Always think about the source population
• Are members of the group easily identifiable?
  Can you interview all or a sample of them?
• Use a cohort study.
• Is the cohort difficult to identify or too large
  to contact all members?
• Use a case-control study design.

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Case-Control or Cohort Which one is right?

• Retrospective cohort study is the most
  appropriate study design here.
• If fewer than 200 people involved, should
  consider interviewing everyone.

Figure 1 Easily identifiable cohort (e.g.,
church picnic, wedding, luncheon)
Not ill n81
Ill n34
Total N 115
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Case-Control or Cohort Which one is right?

• A case-control study could be used for efficiency
  here.
• Or capture entire cohort using e-mail or mail
  surveys.
• Or identify cohorts within the larger cohort
  (e.g., a single dormatory on a college campus).

Figure 2 Easily identifiable but large cohort
(e.g., cruise ship, college campus)
Not ill n2354
Ill n21
Total N 2375
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Case-Control or Cohort Which one is right?
Figure 3 Selecting controls for a rare disease
in a large cohort case-control design and
efficient selection of controls.
16
Case StudiesYersinia and chitterlings

• 11/15/20012/15/2002 12 cases of Yersinia
  enterocolitica identified at large urban
  pediatric emergency department in Tennessee.
• Source population black infants with access to
  medical care from population served by hospital.
• Controls chosen black infants who presented to
  the emergency department of same hospital with
  chief complaint other than gastroenteritis.
• Case-control study implicated source of outbreak
• Chitterlings prepared in 100 of case households
  but only 35 of control households.
• Parents able to identify ways kitchen might have
  become contaminated (e.g., chitterlings cleaned
  in sink).

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Case StudiesPseudomonas from ear piercing

• September 2000 Oregon physician treating 2
  patients on 2 consecutive days with infections of
  the cartilage of the ear both patients received
  ear piercings at same kiosk.
• Investigators could contact all patrons of kiosk
  used a cohort study design
• 118 people received 186 piercings August 1
  through September 15.
• 7 piercings (4) laboratory-confirmed
  Pseudomonas aeruginosa.
• 18 piercings (10) suspected case.
• Risk of infection increased if piercing in
  cartilage rather than earlobe.
• The investigators were able to
• Determine the risk of infection among the entire
  population,
• Determine that the risk was different based on
  site of piercing, and
• Identify practices that might have led to
  contamination of equipment and subsequent
  infection.

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Conclusion

• Cohort and case-control studies are both options
  for determining cause of an outbreak.
• Both study the source population.
• Cohort uses entire population or representative
  sample.
• Case-control uses all cases of disease and
  sampled controls.
• Both types of studies are effective your choice
  will depend on the circumstances of the outbreak
  you are investigating.

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References

1. Dwyer DM, Strickler H, Goodman RA, Armenian HK.
   Use of case-control studies in outbreak
   investigations. Epidemiol Rev. 199416(1)109-123.
   
2. MacDonald PM, Whitwam RE, Boggs JD, et al. 
   Outbreak of Listeriosis among Mexican Immigrants
   as a Result of Consumption of Illicitly Produced
   Mexican-Style Cheese.  Clin Infect Dis. 2005
   40677-682.
3. Jones TF. From pig to pacifier
   chitterling-associated yersiniosis outbreak among
   black infants. Emerg Infect Dis.
   20039(8)1007-1009.
4. Keene WE, Markum AC, Samadpour M. Outbreak of
   Pseudomonas aeruginosa infections caused by
   commercial piercing of upper ear cartilage. Jama.
   2004291(8)981-985.