Epidemiologic-Study Design Dr. Bassam Hijawi Consultant , Epidemiologist MOH , Jordan Define the subject matter Introduce yourself Give housekeeping information (location of restrooms, break policy, etc) Find out any relevant background and interest of the audience

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 . Examples of verbs to use are: Explain Describe Calculate Analyze Prepare Design

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

Epidemiologic Study Design - Observational :. no Hypothesis 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)

Descriptive Studies - Review Populations: - (correlational or ecological studies) Individuals Case reports Case series Cross-sectional surveys Optional slide-delete if irrelevant to your topic

Analytic Studies Observational Cohort studies (prospective) Case-control studies (Retrospective) Experimental (intervention or clinical trial) Optional slide-delete if irrelevant to your topic

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

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?

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

Correlational or Ecological Studies 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

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

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

Case series 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 reports or Case series 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

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

Cross-sectional Surveys (prevalence Study ) 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)

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

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

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) Optional slide-delete if irrelevant to your topic

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

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. Optional slide-delete if irrelevant to your topic

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

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

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

Strengths of cohort studies 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 )

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

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

Analytic Studies Experimental (intervention or clinical trial) Observational Cohort studies (prospective) Case-control studies (retrospective) Optional slide-delete if irrelevant to your topic

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. Optional slide-delete if irrelevant to your topic

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

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

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

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

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

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) Optional slide-delete if irrelevant to your topic

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

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

The cohort study is the gold- standard of analytical Epidemiology

Recap List Describe the differences between and descriptive and analytic studies. Describe the differences between and experimental and observational studies Tip! This should be a repeat of slide two. While this may seem redundant, it emphasizes to the learner what s/he has gained from the session & gives you opportunity to review and catch final questions. Also summarize and make sure they understand where this fits in the ‘big picture’. You can do this in the form of questions. Ex. “So when you are assessing the validity of a test, which of these formulas will you be sure to calculate?”

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

Thank you Dr.Bassam