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# Selecting a Study Design PowerPoint PPT Presentation

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

### 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.

• 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

n=81

Ill

n=34

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

n=2354

Ill

n=21

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.

### 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.

### References

• 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.