Epidemiological study designs

Abdirisak Mohamed Abdi

objectives

• Identify basic study designs used in epidemiology studies.

• Understand the concept of descriptive epidemiological studies.

• Understand the concepts of exposure, outcome and risk.• Identify the association between exposure and

outcome.

introduction

Study design is the arrangement of conditions for the collection and analysis of data to provide the most accurate answer to a question in the most economical way.

Epidemiological study design directs how investigation is conducted .

Epidemiological studies can be classified into descriptive and analytical study designs .

Descriptive and analytical study designs

Descriptive studies involve the systematic collection and presentation of data to give a clear picture of a particular situation and can be carried out on a small or large scale. Descriptive study is used to describes Disease occurrence in a

population e.g. IncidencePrevalenceSurvival

ANALYTICAL STUDIES An Analytical Study attempts to establish causes or risk factors for certain

problems. This is done by comparing two or more groups, some of which have or develop the problem and some of which have not.

Analytical studies is used to determine etiology of disease

Randomized Controlled Trials

Field trials

Community Trials5.Longitudinal

RISK FACTORS

Social

Cultural

Behavioral Biological

Physical

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Association between exposure and outcome

Exposure Outcome

Cause Effect

Descriptive Study Descriptive study is concerned with the following questions:

a. When is the disease occurring (Time)b. Where it is occurring (Place)c. Who is getting the disease (Person)

Descriptive study include:

• Case studies

• Case series

• Ecological /correlation

• Cross sectional

• Longitudinal

Uses of descriptive study Provides data regarding• The magnitude of the disease load• The types of disease problem in the community in term of morbidity

and mortality rate and ratio

Provides “clues” to the disease etiology

Helps in• Formulation of etiological hypothesis

Helps in planning, implementation and evaluation of health services/programmes

Procedures of descriptive study1.Defining the population to be studied• Descriptive study are investigation of population not

individual; include total number• Sex• Age• Occupation• cultural2. Defining the disease under study

• Specified the disease to be investigated

• The main objective to obtain accurate estimate of disease in

a population

3.Describing the disease .

• The main primary objective of descriptive epidemiology is to

describe the occurrence and distribution of disease by

• Time

• Place

• Person4. Measurement of the disease• Cross Sectional Study• Longitudinal Study

• The information should be available in terms of• Mortality• Morbidity (incidence and prevalence)• distribution

5.Comparing with known indicesMaking comparisons between different population, this is to identify groups who are at risk for certain diseases

6. Formulation of an etiological hypothesisHypothesis should formulated in a manner that that it can be tested • Hypothesis should specify the following• -the population- the characteristics of the persons to whom the

hypothesis to be applied• -the specific cause being considered• -the expected outcome• -the dose-response relationship• -the time-response relationship (time between exposure to the

cause observed)

Analyses of descriptive study

• •Data checked for any errors and outliers prior to analysis. • •Data explored graphically e.g. plot the frequency

distributions of various variables• –Check normality of the distribution • •Standard descriptive statistics • –Mean, median, quartiles, mode • –Range, interquartile range, standard deviation • –Standard error and confidence intervals • –Prevalence rates

(cont.) Analysis of descriptive study studies

•Association can be explored using correlation and regression for continuous variables

• –Shows variables are associated, not necessarily imply a cause and effect relationship.

•Means can be compared •Complex multivariate analysis (multiple and logistics

regression) can be carried out • to investigate how a dependent variable is related to

more than one explanatory variable.

Cross sectional studies• Based on a single examination of a cross section of

population at one point in time, results of which can be projected on the whole population provided the sampling has been done correctly.

• In this study design information about the status of an individual with respect to presence/absence of exposure and diseased is assessed at a point in time.

• Cross-sectional studies are useful to generate a hypothesis rather that to test it

• For factors that remain unaltered overtime (e.g. sex, race, blood group) it can produce a valid association

Uses of cross-sectional studya. More useful in chronic diseases b. To find more about disease rather than its etiology• Prevalence• Disease outbreaks• Trends in diseases (repeated c/s studies)• Risk factors for diseases (e.g., NCDs)c. Assessing healthcare needs of populations.d. Describe communitiese. Assess population needsf. Evaluate programsg. Establish baseline data prior to the initiation of longitudinal studies

Advantages of Cross-sectional study

• Cheap, rapid, easy • Can use large sample of the population. • May study several outcomes• Control over selection of subjects• Control over measurements• Relatively short duration• Yields prevalence• Assess health status, and health problems and indicate

priorities for health care planning. Assess customer’s satisfaction for health care.

• Provides the base-line data for further studies if the problem is not studied before.

• Cross sectional study is the most convenient first step in the investigation of the cause of the outbreak or epidemic

Disadvantages of Cross-sectional study

• Does not establish cause/effect ratio

• Potential bias in measuring exposure

• Potential survival bias

• Not feasible for rare disease

• Does not yield incidence

Longitudinal Study

• Based on multiple observations in the same population over a prolong period of time.

Uses of Longitudinal Study

• Natural History of Disease

• Identifying Risk factors

• Finding out incidence rate

Case study

•Detailed presentation of a single case or handful of cases

•Generally report a new or unique finding e.g.: • –previous undescribed disease • –unexpected link between diseases • –unexpected new therapeutic effect • –adverse events •The case may be an individual, an event, a policy, etc

Case study example

• A man suffered from an Myocardial infarction at an young age and survived. What are his questions?– Why did it happen to me?– Can I prevent a second attack?– Are my children at higher risk?– What can be done to lower my risk?– What can be done to lower my children’s risk?

Case report

Case report• unit of study: single person with a

disease• limitation: based on experience of a

single person• provides first clues in the

identification of a disease or adverse effects of exposure e.g (halothane induced hepatitis

Case series

Case series :Unit of study: group of persons with a similar diseases

•Experience of a group of patients with a similar diagnosis •Assesses prevalent disease •Cases may be identified from a single or multiple sources •Generally report on new/unique condition •A realistic design for rare disorders • Retrospective look at series of cases that have features in

common •Common diagnosis, treatment, measures •Each case may be separately described, or the cases may be lumped together with data summaries

Uses of case series Formulation of criteria for diagnosis Formulation of indications for

treatment Identification of prognostic factors Determination of survival rates

Advantages and disadvantages of case series/case study/ case reports

• Advantages •Useful for hypothesis generation •Informative for very rare disease with few established risk factors •Characterizes averages for disorder Disadvantages •Cannot study cause and effect relationships •Cannot assess disease frequency • Sequence of events cannot be ascertained• Not useful for diseases of short period of time• Not practical in studying rare diseases• Case reports are susceptible to bias• Case reports can not be used to make treatment decisions

Ecological study designs

• Ecological study also known as correlation study examine associations between exposure and disease measures on the population level

• In an ecological study, the units of measurements are groups of people rather than individuals.

• The incidence of disease in one geographical area is compared to that of another area. For example a cancer mortality in areas with hazardous waste sites as compared to areas without waste sites.