Epidemiologic study designs THE AGA KHAN UNIVERSITY PAKISTAN

Epidemiologic study designs THE AGA KHAN UNIVERSITY PAKISTAN Dr. Sunita Dodani Assistant Professor Family Medicine, CHS

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

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

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”

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

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

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

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

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

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

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

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

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

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

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.

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

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

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.

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

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

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

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

Epidemiologic study designs What type of study to chose depends on: Ø what is the research question/ objective Ø Ø Ø 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 Ø Choosing an established design gives you a huge head start in design, analysis and eliminating biases