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Selecting a Study Design PowerPoint PPT Presentation

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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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Selecting a Study Design

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Selecting a Study Design


  • 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:

    • 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?

  • 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

  • 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

  • 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 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

  • 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 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

  • 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 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?

  • 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.

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

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 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

  • 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

  • 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.

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