Gastroenteritis at a University in Texas

1
(No Transcript)
2
Session II

• Study Design

3
Session Overview

• Developing and testing hypotheses
• Study Designs
• Selection
• Implementation
• Sampling

4
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

5
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

6
Exposure and Outcome

• A study considers two main factors
• exposure and outcome
• Exposure refers to factors that might influence
  ones 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

7
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

8
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

9
Analytic Studies

• Used to test the current hypothesis
• Is there an association between exposure and
  disease?
• How strong is the association?

10
Analytic Studies

• Two types used in outbreak investigations
• Cohort
• Case-control

11
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

12
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

13
Cohort Studies
Study Population
Exposure is self selected
Non-exposed
Exposed
Follow through time
Disease
No Disease
No Disease
Disease
14
Cohort Study

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

15
Cohort StudiesProspective vs. Retrospective
Exposure Outcome
Prospective Assessed at beginning of study Followed into the future for outcome
Retrospective Assessed at some point in the past Outcome has already occurred
16
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

17
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
18
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.

19
Case-Control Study

• Sometimes, identifying a cohort is difficult
• Members of cohort cant be identified / contacted
• Case-control study is alternative

20
Case-control Studies
No Exposure
No Exposure
Had Exposure
Had Exposure
Cases
Controls
Study Population
21
Case-Control Study

• Steps in a Case-Control Study
• Identify the source population
• Establish a case definition and select cases
• Select controls

22
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

23
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

24
Case-Control Study

• Step 3 Select controls
• Represent source population
• Collect same exposure information as for cases

25
Case-Control Study

• Step 3 Select controls (contd.)
• Sources of controls
• Random sample
• Friends of cases

26
Cohort versus Case-Control
27
Study Design Advantages Disadvantages
Cohort Least prone to selection bias Can reasonably conclude that cause preceded disease Can study several diseases at once Can examine rare exposures Retrospective can be low-cost Prospective can be expensive, time-consuming Prospective can lead to loss to follow up Exposed may be followed more closely than unexposed, yielding invalid conclusions about causality
Case- Control Less expensive and quicker than cohort Can examine the effect of multiple exposures Require a smaller sample population Inefficient for studying rare exposures Susceptible to selection bias Cannot directly estimate the risk of disease Cannot study several diseases at once
28
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

29
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

30
Sampling

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

31
Sampling

• Why sample?
• Because it is more efficient saves time and
  money!

32
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.

33
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.

34
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

35
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.

36
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.

37
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/classroo
  m/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.

38
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

39
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
  Instructors Guide Analytic Study Designs. Epid
  168 lecture series. http//www.epidemiolog.net/epi
  d168/labs/AnalyticStudExerInstGuid2000.pdf