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Session II. Study Design. Session Overview. Developing and testing hypotheses Study Designs: Selection Implementation Sampling. Learning Objectives. Understand the differences in methodology between various study designs

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Session II

Study Design


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Session Overview

  • Developing and testing hypotheses

  • Study Designs:

    • Selection

    • Implementation

  • Sampling


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Learning Objectives

  • Understand the differences in methodology between various study designs

  • Be able to describe the advantages and disadvantages of alternative study designs

  • Know how to assess which study design to apply during an outbreak investigation

  • Understand how to select cases and controls in a case-control study design

  • Understand sampling


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Basic Steps of an Outbreak Investigation

  • Verify the diagnosis and confirm the outbreak

  • Define a case and conduct case finding

  • Tabulate and orient data: time, place, person

  • Take immediate control measures

  • Formulate and test hypothesis

  • Plan and execute additional studies

  • Implement and evaluate control measures

  • Communicate findings


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Exposure and Outcome

A study considers two main factors:

exposure and outcome

  • Exposure refers to factors that might influence one’s risk of disease

    • Smoking

    • Eating at a particular restaurant

  • Outcome refers to case definitions

    • Individuals who do and do not have the disease/condition of interest


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Developing Hypotheses

  • A hypothesis is an educated guess about an association that is testable in a scientific investigation

  • Descriptive data provide information to develop hypotheses

  • Hypotheses tend to be broad initially and are then refined to have a narrower focus


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Example

  • Hypothesis: People who ate at the church picnic were more likely to become ill

    • Exposure is eating at the church picnic

    • Outcome is illness - diarrhea and fever, where diarrhea is defined as at least 3 soft stools in a 24 hour period

  • Hypothesis: People who ate the egg salad at the church picnic were more likely to have laboratory-confirmed Salmonella

    • Exposure is eating egg salad at the church picnic

    • Outcome is laboratory confirmation of Salmonella


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Analytic Studies

  • Used to test the current hypothesis:

    • Is there an association between exposure and disease?

    • How strong is the association?


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Analytic Studies

Two types used in outbreak investigations

  • Cohort

  • Case-control


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Definition of a Cohort

In epidemiology, “Any designated group of individuals who are followed or traced over a period of time.”

- Last, JM. A Dictionary of Epidemiology, 3rd ed. New York: Oxford University Press, 1995


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Cohort Study Types

A cohort study analyzes an exposure / disease relationship within the entire cohort.

  • Prospective

    • The Framingham Study

  • Retrospective

    • Usually used in outbreak investigations


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Cohort Studies

Study

Population

Exposure is

self selected

Non-exposed

Exposed

Follow through

time

Disease

No Disease

Disease

No Disease


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Cohort Study

  • Identify cohort

    • Do not select cohort so that either everyone is exposed or everyone is diseased


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Cohort Studies:Prospective vs. Retrospective


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Cohort Study

  • Preferred study design when:

    • Members of cohort are easily identifiable

    • Members of a cohort are easily accessible

    • Exposure is rare

    • There may be multiple diseases involved


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Cohort Study Example

  • Recent norovirus outbreaks on cruise ships

  • Attempt to interview all passengers

  • Collect food history information

MMWR: December 13, 2002 / 51(49);1112-1115


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Cohort Study Examples

  • Shigellosis among swimmers in a Georgia park

    • Used park registry to identify park visitors

      Iwamoto M, Hlady G, Jeter M et al. Shigellosis among Swimmers in a Freshwater Lake-Georgia, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference. Atlanta, GA. April, 2004.

  • Whirlpools and Methicillin-Resistant Staphylococcus aureus

    • Occurred on a college football team

      Begier EM, Barrett FK, Mshar PA et al. Body Shaving, Whirlpools, and Football: An Out break of Methicillin-Resistant Staphylococcus aureus Cutaneous Infections in a College Football Team-Connecticut, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference. Atlanta, GA. April, 2004.


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Case-Control Study

  • Sometimes, identifying a cohort is difficult

    • Members of cohort can’t be identified / contacted

  • Case-control study is alternative


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Case-control Studies

Had Exposure

No Exposure

Had Exposure

No Exposure

Cases

Controls

Study

Population


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Case-Control Study

Steps in a Case-Control Study:

  • Identify the source population

  • Establish a case definition and select cases

  • Select controls


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Case-Control Study

Step 1 - Identify source population

  • Represents the population that the cases came from; is similar to the cohort in a cohort study


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Case-Control Study

Step 2 – Establish a case definition

and select cases

  • A standard set of criteria for deciding disease status

    • Clinical criteria, time, place, and person


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Case-Control Study

Step 3 – Select controls

  • Represent source population

  • Collect same exposure information as for cases


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Case-Control Study

Step 3 – Select controls (cont’d.)

  • Sources of controls

    • Random sample

    • Friends of cases



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Matching in Case-Control Studies

  • Makes one or more case and control attributes similar (e.g., age, gender, residence)

  • An unmatched study design is usually preferred


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Matching: Points to Consider

  • More complex data analysis required

  • Inability to assess role of matching factor on disease status

    • Do not match on exposure factor

  • Potential for over-matching


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Sampling

Sampling is the systematic selection of a portion of the larger source population. A sample should be representative of the larger source population.


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Sampling

Why sample?

Because it is more efficient – saves time and money!


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Sampling

Sample size

Is the purpose of the study to determine the source of the outbreak?

  • A small number of cases and controls can reveal risk factors for infection.

    Is the purpose of the study to determine the number of persons who become sick over a specific period of time?

  • A cohort study would require a larger sample.


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Sampling

Types of sampling

Simple random sample (SRS)

Randomly select persons to participate in study. There are many variations of SRS.

Convenience sample

Choose those individuals who are easily accessible.


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Sampling

Problems with convenience sampling

  • Based on subjective judgment

  • Cases may or may not be representative of the total population

  • May lead to biased results


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Session II Summary

  • An analytic study is used to test scientific hypotheses that may help support actions for specific control measures and to help prevent recurrence of a problem.

  • A case definition with specific criteria helps you select your study population, as long as it does not include the hypothesis.

  • Case-control studies, when conducted properly, are generally adequate and usually more efficient than cohort studies.


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Session II Summary

  • Cohort studies may be preferable when you work with confined (e.g., easily identifiable and accessible) study populations such as on a cruise ship or at a wedding reception.

  • Case-control study controls need to be representative of the source population, and not matched on the exposure factor if matching is used.


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References and Resources

  • Begier EM, Barrett FK, Mshar PA et al. Body Shaving, Whirlpools, and Football: An Out break of Methicillin-Resistant Staphylococcus aureus Cutaneous Infections in a College Football Team-Connecticut, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference. Atlanta, GA. April, 2004.

  • Centers for Disease Control and Prevention (1992). Principles of Epidemiology: 2nd Edition. Public Health Practice Program Office: Atlanta, GA.

  • Centers for Disease Control and Prevention "Gastroenteritis at a University in Texas" http://www.phppo.cdc.gov/phtn/casestudies/classroom/gastro.htm

  • Gordis, L. (2000). Epidemiology: 2nd Edition. W.B. Saunders Company: Philadelphia, PA.

  • Gregg, M.B. (2002). Field Epidemiology: 2nd Edition. Oxford University Press: New York.

  • Hennekens, C.H. and Buring, J.E. (1987). Epidemiology in Medicine. Little, Brown and Company: Boston/Toronto.


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References and Resources

  • Iwamoto M, Hlady G, Jeter M et al. Shigellosis among Swimmers in a Freshwater Lake-Georgia, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference. Atlanta, GA. April, 2004.

  • Kleinbaum, D., Sullivan, K., and Barker, N. (2003). ActivEpi Companion Textbook. Springer-Verlag: New York.

  • Last, J.M. (2001). A Dictionary of Epidemiology: 4th Edition. Oxford University Press: New York.

  • McNeill, A. (January 2002). Measuring the Occurrence of Disease: Prevalence and Incidence. Epid 160 lecture series, UNC Chapel Hill School of Public Health, Department of Epidemiology.

  • Morton, R.F, Hebel, J.R., McCarter, R.J. (2001). A Study Guide to Epidemiology and Biostatistics: 5th Edition. Aspen Publishers, Inc.: Gaithersburg, MD.

  • North Carolina Center for Public Health Preparedness. March 2005 Public Health Information Network session: “Descriptive and Analytic Epidemiology.”

    http://www.sph.unc.edu/nccphph/phtin/index.htm


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References and Resources

  • University of North Carolina at Chapel Hill School of Public Health, Department of Epidemiology, and the Epidemiologic Research & Information Center (June 1999). ERIC Notebook. Issue 2. http://www.sph.unc.edu/courses/eric/eric_notebooks.htm

  • University of North Carolina at Chapel Hill School of Public Health, Department of Epidemiology, and the Epidemiologic Research & Information Center (July 1999). ERIC Notebook. Issue 3. http://www.sph.unc.edu/courses/eric/eric_notebooks.htm

  • University of North Carolina at Chapel Hill School of Public Health, Department of Epidemiology, and the Epidemiologic Research & Information Center (September 1999). ERIC Notebook. Issue 5. http://www.sph.unc.edu/courses/eric/eric_notebooks.htm

  • University of North Carolina at Chapel Hill School of Public Health, Department of Epidemiology (August 2000). Laboratory Instructor’s Guide: Analytic Study Designs. Epid 168 lecture series. http://www.epidemiolog.net/epid168/labs/AnalyticStudExerInstGuid2000.pdf


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