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

Epidemiologic study Epidemiologic study designsdesigns

Dr. Sunita Dodani

Assistant Professor

Family Medicine, CHS

THE AGA KHAN THE AGA KHAN UNIVERSITYUNIVERSITYPAKISTANPAKISTAN

Learning ObjectivesLearning Objectives

• To understand the concepts of different study designs

• To learn about the advantages and disadvantages of several study designs

Performance ObjectivesPerformance 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 designsEpidemiologic 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”


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


Types of StudiesA. Experimental - study factor is manipulated by

the investigatorTypes

1. Pure versus Quasi-experimental2. Laboratory versus real world

B. Observational - no manipulation of study factor by the investigator1. Descriptive versus Analytic2. Retrospective versus Prospective


Factors Important in Study Design

A. Specific, testable hypotheses - NOT a fishing expedition

B. Biases

1. Internal validity

2. External validity


Study designsCase series

• Case Series report new diseases or health related problems.

• They may provide some descriptive data on exposures to potential causal factors


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


Cross Sectional Studies (contd)

• sample without knowledge of Exposure or Disease

• sample at one point in time

• Mostly prevalence studies/surveys



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



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


Cross Sectional StudiesDisadvantages• 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


Cross Sectional StudiesDisadvantages• 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


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.


Case-control studiesAdvantages• 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


Case-control studiesDisadvantages• 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


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.


Cohort StudiesAdvantages

• 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


Cohort StudiesAdvantages

• 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


Cohort studiesDisadvantages

• 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


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


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