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Epidemiologic study designs Dr. Sunita Dodani Assistant Professor Family Medicine, CHS THE AGA KHAN UNIVERSITYPAKISTAN To insert your company logo on this.

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Presentation on theme: "Epidemiologic study designs Dr. Sunita Dodani Assistant Professor Family Medicine, CHS THE AGA KHAN UNIVERSITYPAKISTAN To insert your company logo on this."— Presentation transcript:

1 Epidemiologic study designs Dr. Sunita Dodani Assistant Professor Family Medicine, CHS THE AGA KHAN UNIVERSITYPAKISTAN To insert your company logo on this slide From the Insert Menu Select “Picture” Locate your logo file Click OK To resize the logo Click anywhere inside the logo. The boxes that appear outside the logo are known as “resize handles.” Use these to resize the object. If you hold down the shift key before using the resize handles, you will maintain the proportions of the object you wish to resize.

2 Learning Objectives To understand the concepts of different study designs To learn about the advantages and disadvantages of several study designs

3 Performance Objectives After this lecture the student will be able To recognize concepts of different study designs To appropriately use a study design in research projects

4 Epidemiologic study designs The basis for the lecture is the distinction between descriptive epidemiology and analytic epidemiology Descriptive epidemiology: seeks to measure the frequency in which diseases occur or collect descriptive data on possible causal factors. Analytic epidemiology: attempts to specify in more detail the causes of a particular disease”

5 Epidemiologic study designs Types of Epidemiologic Observational Studies based on: type of sampling from population - based on Exposure &/or Disease or neither temporal sequence of observation - one time point, forward, backwards

6 Epidemiologic study designs Types of Studies A. Experimental - study factor is manipulated by the investigator Types 1. Pure versus Quasi-experimental 2. Laboratory versus real world B. Observational - no manipulation of study factor by the investigator 1. Descriptive versus Analytic 2. Retrospective versus Prospective

7 Epidemiologic study designs Factors Important in Study Design A. Specific, testable hypotheses - NOT a fishing expedition B. Biases 1. Internal validity 2. External validity

8 Epidemiologic study designs Study designs Case series Case Series report new diseases or health related problems. They may provide some descriptive data on exposures to potential causal factors

9 Epidemiologic study designs Cross-sectional studies Cross-Sectional Studies measure existing disease and current exposure levels. They provide some indication of the relationship between the disease and exposure or non-exposure

10 Epidemiologic study designs Cross Sectional Studies (contd) sample without knowledge of Exposure or Disease sample at one point in time Mostly prevalence studies/surveys

11 Epidemiologic study designs Cross Sectional Studies (contd) Advantages Good design for hypothesis generation Can estimate overall and specific disease prevalence and sometimes rates Can estimate exposure proportions in the population Can study multiple exposures or multiple outcomes or diseases

12 Epidemiologic study designs Cross Sectional Studies (contd) Advantages Relatively easy, quick and inexpensive No issue of subjecting any animals or producers to particular treatments Best suited to studying permanent factors (breed, sex, blood-type) Often good first step for new study issue

13 Epidemiologic study designs Cross Sectional Studies Disadvantages Impractical for rare diseases Not a useful type of study for establishing causal relationships Confounding is difficult to control No control over sample size for each exposure by disease subclass

14 Epidemiologic study designs Cross Sectional Studies Disadvantages Problems with temporal sequence of data hard to decide when disease was actually acquired disease may cure the exposure miss diseases still in latent period recall of previous exposure may be faulty

15 Epidemiologic study designs Case-control studies Case-Control Studies identify existing disease/s and look back in previous years to identify previous exposures to causal factors. Cases are those who have a disease. Controls are those without a disease. Analyses examine if exposure levels are different between the groups.

16 Epidemiologic study designs Case-control studies Advantages Best design for rare diseases Can be accomplished quickly since events of interest have already occurred Can study several potential exposures at the same time Lends itself well to hospital-based studies and outbreaks

17 Epidemiologic study designs Case-control studies Disadvantages Problems with temporal sequence of data Hard to decide when disease was actually acquired Disease may cure the exposure Miss diseases still in latent period Can’t calculate incidence, population relative risk or attributable risk HIGH potential for bias

18 Epidemiologic study designs Cohort Studies In a cohort study, subjects with an exposure to a causal factor are identified and the incidence of a disease over time is compared with that of controls (persons who do not have the exposure). In a longitudinal study, subjects are followed over time with continuous or repeated monitoring of risk factors or health outcomes, or both.

19 Epidemiologic study designs Cohort Studies Advantages Can get best assessment of exposure and can deal with changes in exposure May be only design if exposure needs to be measured directly Allows study of rare exposures Only way to get prospective information for rapidly fatal diseases Good for establishing temporal sequence and natural history of disease Can examine multiple outcomes linked to exposure - often find other effects than

20 Epidemiologic study designs Cohort Studies Advantages Can estimate overall and specific disease rates, usually incidence Researcher selects, measures & records data Lower potential for bias than a case-control study - no recall and selection bias Results are considered more conclusive than results from case-control studies The longer a cohort study continues, the stronger it can become

21 Epidemiologic study designs Cohort studies Disadvantages Impractical for rare diseases - even if risk is high, no cases may occur or results may not be statistically significant Larger sample size than case-control Data collection is usually very expensive Long time commitment for follow-up

22 Epidemiologic study designs Other designs: Time series - test if incidence of disease changes in a population over time Hybrid - often what is seen in practice Can be efficient and match necessity Can lead to bias and disaster Meta-analysis Combining results from a range of published studies Established methodology, not just literature review

23 Epidemiologic study designs What type of study to chose depends on:  what is the research question/ objective  Time available for study  Resources available for the study  Common/rare disease or production problem  Type of outcome of interest  Quality of data from various sources  Often there are multiple approaches which will all work  Choosing an established design gives you a huge head start in design, analysis and eliminating biases


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