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:

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 identif iable 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 eff icient 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.

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.