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Epidemiologic-Study Design

Epidemiologic-Study Design. Dr. Bassam Hijawi Consultant , Epidemiologist MOH , Jordan. By the end of this presentation , you will be able to know about : Different types of Epid. Study design (Research ) Differences between Descriptive & Analytical design

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Epidemiologic-Study Design

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  1. Epidemiologic-Study Design Dr. BassamHijawi Consultant , Epidemiologist MOH , Jordan

  2. By the end of this presentation , you will be able to know about : • Different types of Epid. Study design (Research ) • Differences between Descriptive & Analytical design • Differences between types of Analytical design .

  3. Epidemiologic - Study Design • Descriptive epidemiologic studies. • Analytical epidemiologic studies.

  4. Epidemiologic Study Design- Observational : .no Hypothesis *cross Sectional Survey (prevalence Survey ) .Hypothesis *Analytical Studies *Ecological Studies -Experimental(unethical to perform experiments on people if exposure is harmful)

  5. Descriptive Studies - Review • Populations: - (correlational or ecological studies) • Individuals • Case reports • Case series • Cross-sectional surveys

  6. Analytic Studies • Observational • Cohort studies (prospective) • Case-control studies (Retrospective) • Experimental (intervention or clinical trial)

  7. Descriptive Studies • Populations (correlational or ecological studies) • Individuals • Case reports • Case series • Cross-sectional surveys

  8. Correlational or Ecological Studies • It is type of descriptive study • Based on aggregate measures of exposure and outcome from several populations. • The population is the unit of observation available for study. • Exposures: • What percent of a population smokes? • What percent of 1-year old children are vaccinated against measles? • What percent of a population has piped water?

  9. Based on aggregate measures of exposure and outcome from several populations. Outcome: What percentage of a population died from MI? What percentage of children had measles last year? What percentage of population had episodes of diarrhea? Correlational or Ecological Studies

  10. Advantages Easy to do Use available data (“administrative” or other aggregate data) Generate hypotheses for additional study Correlational or Ecological Studies

  11. Disadvantages Unable to examine data for individuals No assurance that persons with exposure are the same ones with the outcome Ecologic fallacy Unable to adjust for confounding factors Correlational or Ecological Studies

  12. Populations (correlational or ecological studies) . Individuals Case reports Case series Cross-sectional surveys Descriptive Studies

  13. The individual is the unit of observation Clinical case with “unusual” clinical picture May suggest an etiological association Case reports

  14. First case report may stimulate compilation of additional case reports….a case series (e.g. occurrence of Pneumocystis carinii among a group of young, homosexual men with no history of immune deficiency) Case series

  15. Advantages: Use available clinical data Detailed individual data Suggest need for investigation (hypothesis generation) Disadvantages: May reflect experience of one person or one clinician No explicit comparison group Case reports or Case series

  16. Populations (correlational or ecological studies) Individuals Case reports Case series Cross-sectional surveys Descriptive Studies

  17. Collection of data on several individuals at “one point” in time. Examples: national census community survey survey of a particular group of persons (e.g., occupational group, school children) Cross-sectional Surveys(prevalence Study )

  18. N____S____ Flow Diagram of C.S Study EC EC EC EC

  19. Advantages Standardized data collection tool. Able to focus data collection in specific locations or specific groups of persons. May make comparsions among study participants. Relatively quick to do. May be repeated to get data on trends. Cross-sectional Surveys

  20. Cross-sectional Surveys Disadvantages • May be biased by lack of participation • Reflects prevalent, not incident cases

  21. Analytic Studies • Experimental (intervention or clinical trial) • Observational • Cohort studies (Follow up , Incidence , Panel or prospective ) • Case-control studies (case – referent, case-history , trohoc or etrospective)

  22. What is a cohort?Marching towards outcomes • Group of individuals • sharing same experience • followed up for specified period of time • Examples • birth cohort • cohort of guests at wedding reception • Jordan FETP fourth cohort

  23. Cohort Study • Investigator selects individuals on the basis of exposure status. • Exposure may have begun in past. • Investigator “follows up” the study participants to record occurrence of a particular event (outcome). • Compare rates of disease occurrence among exposed and unexposed groups of persons.

  24. Design of an Cohort Study Individuals “choose” their exposure status Source: partially adapted from WHO, 1993

  25. Terminology: Retrospective or Prospective? • Suggest use the terms “retrospective” or “prospective” to refer to the timing of events in relation to initiation of study. (Hennekens and Buring, 1987) Retrospective cohort study: exposure and disease have occurred prior to start of study • Prospective cohort study: disease has not occurred prior to start of study

  26. Cohort Study: Advantages • Calculate incidence rates (attack rates) • Clear definition of temporal sequence • Well-suited to study result of a rare exposure • Able to identify adequate numbers of exposed and unexposed persons

  27. Strengths of cohort studies*can directly measure -incidence in exposed and unexposed groups -true relative risk *well suited for rare exposure*temporal relationship exposure- disease is clear*less subject of selection biases outcome Not known (prospective )

  28. Cohort Study: Advantages Allows examination of multiple effects of exposure • If prospective, minimizes bias in: • Selection, since enrollment is completed without knowledge of disease outcome • exposure measurement

  29. Cohort Study: Disadvantages • Inefficient for diseases with long latency period • If prospective, expensive and time consuming • If retrospective, requires availability of adequate records. • Loss of participants during follow-up period • Not suitable for rare diseases (large sample) • Exposure can change • Multiple exposure = difficult • Expensive and time consuming

  30. Analytic Studies • Experimental (intervention or clinical trial) Observational • Cohort studies (prospective) • Case-control studies (retrospective)

  31. Case-control Study • Investigator selects individuals on the basis of disease status. • May use incident or prevalent cases • Investigator assess the exposure history of study participants. • Compare odds of exposure among persons with disease to odds of exposure among persons without disease.

  32. Design of a Case-control Study Source: partially adapted from WHO, 1993

  33. Case-control Study: Advantages • Well-suited to study result of rare diseases • Study of diseases with a long latency period • Efficient use of time and resources • Able to identify adequate numbers of affected and unaffected persons • Able to study a wide range of potential etiologic exposures

  34. Case-control Study: Disadvantages *cant compute directly relative risk *not suitable for rare exposure *temporal relationship exposure- disease difficult to establish. *loss of precision due to sampling *Not efficient for rare exposures *biases +++ control selection recall bias when collecting data

  35. What kind of study design would you use in an outbreak investigation? • Cohort? Why? • Case-control? Why? • Other design ? Why ?

  36. Choice of study design depends on: • Nature of disease under investigation • Type of exposure • Available resources

  37. Experimental (intervention or clinical trial) • Investigator assigns determines who is exposed • Investigator “follows up” the study participants to record occurrence of a particular disease (or clinical treatment outcome)

  38. Experimental (intervention or clinical trial) • Could be based on individual or community level • Exposure could be assigned in a random or a non-random manner • Compare rates of outcome

  39. Design of an Experimental Study Investigator Determines Exposure Status Source: partially adapted from WHO, 1993

  40. The cohort study is the gold- standard of analytical Epidemiology

  41. Recap • List Describe the differences between and descriptive and analytic studies. • Describe the differences between and experimental and observational studies

  42. Recap • Describe different types of observational studies. • Describe the advantages and disadvantages descriptive and analytical studies.

  43. Thank youDr.Bassam

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