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Selecting a Study Design. 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. About Analytic Studies. We can use analytic studies to test hypotheses. We want to know:

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  • Describe the cohort study design.
  • Describe the case-control study design.
  • Compare situations in which cohort and case-control study designs should be used.
about analytic studies
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.
what is a cohort
What is a Cohort?
  • A “cohort” is a group of people who have something in common.
  • Can represent the source population—the 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
cohort studies
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.
identifying a cohort
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.
identifying a cohort7
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.
case control studies
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.
case control studies9
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.
selecting cases and controls
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.
selecting cases and controls11
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.
case control or cohort which one is right
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.
case control or cohort which one is right13
Retrospective cohort study is the most appropriate study design here.

If fewer than 200 people involved, should consider interviewing everyone.

Case-Control or Cohort: Which one is right?

Figure 1: Easily identifiable cohort (e.g., church picnic, wedding, luncheon)

Not ill




Total N = 115

case control or cohort which one is right14
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).

Case-Control or Cohort: Which one is right?

Figure 2: Easily identifiable but large cohort (e.g., cruise ship, college campus)

Not ill




Total N = 2375

case control or cohort which one is right15
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.

Hispanic residents in a county

Female Hispanic residents in a county

Female Hispanic residents of child-bearing age in a county

Pregnant Hispanic residents in a county

12 cases of listeriosis among pregnant Hispanic women

case studies yersinia and chitterlings
Case Studies:Yersinia and chitterlings
  • 11/15/2001–2/15/2002: 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).
case studies pseudomonas from ear piercing
Case Studies:Pseudomonas 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.
  • 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.
  • Dwyer DM, Strickler H, Goodman RA, Armenian HK. Use of case-control studies in outbreak investigations. Epidemiol Rev. 1994;16(1):109-123.
  • 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; 40:677-682.
  • Jones TF. From pig to pacifier: chitterling-associated yersiniosis outbreak among black infants. Emerg Infect Dis. 2003;9(8):1007-1009.
  • Keene WE, Markum AC, Samadpour M. Outbreak of Pseudomonas aeruginosa infections caused by commercial piercing of upper ear cartilage. Jama. 2004;291(8):981-985.