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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: - PowerPoint PPT Presentation
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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:
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.
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.
Not illn=81
Illn=34
Total N = 115
Figure 1: Easily identifiable cohort (e.g., church picnic, wedding, luncheon)
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).
Not illn=2354
Illn=21
Total N = 2375
Figure 2: Easily identifiable but large cohort (e.g., cruise ship, college campus)
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 countyPregnant 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.
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.
References1. Dwyer DM, Strickler H, Goodman RA, Armenian HK. Use
of case-control studies in outbreak investigations. Epidemiol Rev. 1994;16(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; 40:677-682.
3. Jones TF. From pig to pacifier: chitterling-associated yersiniosis outbreak among black infants. Emerg Infect Dis. 2003;9(8):1007-1009.
4. 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.