This document discusses different types of epidemiological studies including descriptive studies, analytical studies, and experimental studies. Descriptive studies are divided into population studies and individual studies. Analytical studies include case-control studies and cohort studies. Key aspects of case-control and cohort study designs such as selection of cases/controls, sources of information, issues in analysis/interpretation, and strengths/weaknesses are described.

1. Dr. Akhilesh BhargavaMD, DHA, PGDHRMProf. Community Medicine &Director-SIHFW, JaipurEpidemiological studiesAkhilesh Bhargava1

2. Akhilesh Bhargava2Epidemiological studiesDescriptiveCorrelation studiesIndividual studiesAnalyticalCase control studiesCohort studiesExperimentalRandomized designBlindDouble blindTriple blindClinical trials

3. Akhilesh Bhargava3Types of Descriptive studiesa. Population (Correlation) studies.b. Individual studiesi. Case reports ii. Case series iii. Cross sectional studies (Prevalence studies)

4. Akhilesh Bhargava4Population (Correlation) studies.Use data from entire population to compare-Disease frequency between different population groups during same periodDisease frequency insame population at different periods.useful in formulation of a hypothesis but not for testing a hypothesis.

5. Akhilesh Bhargava5Advantages and LimitationsExposure cannot be linked with disease as whole population is represented.Lack of ability to control the effects of potential confounding variables.Presence of a correlation does not necessarily mean a statistical association.Correlation data represent average exposure levels rather than actual individual levels Inexpensive in terms of Time & MoneyRoutinely available information can be used

6. Akhilesh Bhargava6Individual studiesCase reports (single patient)Case series (group of patients with similar diagnosis)

7. Akhilesh Bhargava7Individual studies:Advantages and LimitationsCases can be aggregated from different sources to generate hypotheses and describe syndromesStatistical association can not be established as there is no comparison group

8. Akhilesh Bhargava8Cross sectional studiesMeasure disease and exposure simultaneously in a defined population over a defined periodprovide instant information about frequency and characteristics of a diseaseUseful in-Assessing health statusIdentifying health care needs.Providing data on Prevalence of Disease, disability and utilization of servicesProvide data for Health care planning and administration

9. Akhilesh Bhargava9Advantages and LimitationsShort term so less expensiveOffer starting point in prospective studiesAllow assessment of risk though less preciseStart from a reference population from where sample is drawn, generalization can be madeNot possible to ascertain whether exposure preceded or resulted from the disease.Since prevalence is assessed, results are affected by survival factors

10. Akhilesh Bhargava10Case-Control studyA non-experimental Study, Subjects enrolled based on presence/absence of outcome,Cases/controls compared with regard to prior exposure to causal factors.

11. Akhilesh Bhargava11Designing: case-controlCriteria- Comparability of cases & controlsIssues – # Defining & selection of cases # Selection of controls # Information on disease exposure status

12. Akhilesh Bhargava12Case-control: strengths &limitationsStudy of diseases with long latent periodLow costShort timeApplicable to rare diseases alsoBias in selection/Reporting/Recording.

13. Akhilesh Bhargava13Case-control : selection of casesRequirements-A standard case definitionInclusion of likes, exclusion of dislikesA strict diagnostic criteria for homogeneityOnly newly diagnosed cases be included- Older cases may distort the presentationSelection –Hospital based Pop. based

14. 14Group of interest eg. Cancer patientsTake historiesDraw conclusionsCompare historiesTake historiesComparison group eg.Non-patientsCase Control StudiesAkhilesh Bhargava

15. Akhilesh Bhargava15Cases ControlsSelection criteriaSources Gen. Population

16. Hospital Patients

17. Whole group

18. SampleSelection criteriaSources Hospital (bias ?)

19. Relative (genetic)

20. Neighbors

21. Gen. populationAkhilesh Bhargava16Control selection criteriaCharacteristics and sources of Cases Need to obtain comparable information from cases and controls Practicality Cost Representative of population from where cases are drawn

22. Akhilesh Bhargava17Advantages/Disadvantage of control sourcesSource AdvantageDisadvantageHospital - easy identification - do not represent - belong to same class exposure in general - cooperative population - min. non-response - admission bias Gen .Pop. - high comparability - costly, more time, - recall bias - loss during study

23. Akhilesh Bhargava18Selection of controlsHow many control groups ?Hospital – more than oneWhere one group has some deficiency (stress & peptic ulcer in executives) and stress present in hospitalizedHow many subjects in a control group?1:1 if no. of cases & controls is large and cost of getting information is sameMax. 4:1, more does not increase statistical strength but cost increases

24. Akhilesh Bhargava19Case Control Study ApproachOdds Ratio = Odds of exposure in cases Odds of exposure in controls a / cb/ da db c==

25. Akhilesh Bhargava20Odds Ratio = Relative RiskConditions & InterpretationsOR –an unbiased and valid estimate of RR inCase control studies, as- only newly diagnosed are included selection is not based on exposure level An OR=1- no statistical association - exposure is not a risk factor OR >1 – a positive association between exposure and outcome OR<1 – Less risk, or even protective value of a Risk factor.

26. Akhilesh Bhargava21Bias and its play in Case Control study-Bias “any systematic error in the study that results in an incorrect estimate of the association between exposure and risk of disease”.

27. Akhilesh Bhargava22Types of Bias-1. Selection Bias a. Prevalence-Incidence Bias b. Admission rate (Berkson’s) Bias c. Non-response/ Refusal Bias2. Observational or Information Bias a. Diagnostic Bias b. Recall Bias

28. Akhilesh Bhargava23How Bias gets inDifferential surveillance,

29. Diagnosis & Referral during ascertaining status of subjects, OR , for-Unwillingness to participate ,Non selection by investigator &

30. Replacement of originally selected

31. Selection bias- Refusal Non-response Self selection

32. Cohortgroup with common experience Roman Army GroupsCohort study-“A non-experimental study, subjects enrolled based on exposure level to main independent variable, followed to determine development of the dependent variable”24.Akhilesh Bhargava

33. Cohort studyCharacteristics- Absence of disease at initiation point

34. Follow up over a period of timeTypes-Retrospective (after exposure but certainly before disease/ outcome)Prospective (exposure may/may not have but outcome certainly not)

35. ExposureCase control studyDisease ?---------------------------------- ?---------------------------------- Prospective Cohort StudyExposure Disease --------------- ------------------? ------------------- ------------------- ? Retrospective cohort StudyExposure Disease----------------------------------------------- ? ---------------------------------------------- ?

36. Study design- considerations General- Hypothesis to be tested Resources Current state of knowledge Which of cohort study ? Time Money Latent pd. of disease Availability of information/Records Frequency of occurrence of disease Sample size Follow up period required

37. Issues in Cohort study design Selection – Exposed group Comparison group Sources of data – Exposure data Outcome data Approaches to follow-up

38. Cohort study…Selection-Exposed/Comparison groups Exposed Criteria- Availability Complete & accurate information Exposure Comparison Criteria- Similarity but for exposure

39. Cohort…Sources of information Information- complete & accurateExposed- Gen. population Special exposure groupsComparison- Graded exposure groups Individuals from same work cohort without exposure to risk factor Rates of disease in gen. pop. Gen. Population

40. Cohort…Sources of exposure information Medical records

41. Employment records