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![WHAT IS COHORT
Ancient Roman
military unit, A band
of warriors.
Persons banded
together.
Group of persons with
a common statistical
characteristic. [Latin]
E.g. age, birth date,
Jan 14, 2018 49](https://image.slidesharecdn.com/analyticalepidemiology-180114110843/75/Analytical-epidemiology-49-2048.jpg)
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Analytical epidemiology
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- 2. What is epidemiology? Jan 14, 2018 2
- 3. Defined by JohnM. Last in 1988 “Study of Distribution and Determinants of health related state or event in a specified population and the application of this study to the control of health problem”. We measure – – Disease frequency – Diseases distribution – Determinants of disease. Jan 14, 2018 3
- 4. TYPES OF EPIDEMIOLOGICALSTUDIES 1. OBSERVATIONAL STUDIES A. DESCRIPTIVE STUDY DESCRIBE DIESEASE BY TIME PLACE PERSON B. ANALYTICAL STUDIES ECOLOGICAL STUDY CROSS SECTIONAL STUDY CASE-CONTROL STUDY COHORT STUDY 2. EXPEREMENTAL STUDIES RANDOMIZED CONTROLLED TRIAL (RCT) FIELD TRIAL COMMUNITY TRIAL Jan 14, 2018 4
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- 6. AIMS OF EPIDEMIOLOGY to describe the size and distribution of the disease problems in human populations. to provide the data essential for the planning, implemnetation and evaluation of health services for the prevention, control and treatment of diseases and for the setting up of priorities among those services. Jan 14, 2018 6
- 7. to identifyetiological factors in the pathogenesis of disease. Jan 14, 2018 7
- 8. PRINCIPLES OF EPIDEMIOLOGY exact observation (strict, vigrous, accurate, precise) Correct interpretation( free from error) rational explanation (intelligent, sensible , reasonable) scientific consteuction( by expert knowledge and technical skill) Jan 14, 2018 8
- 9. TOOLS OF MEASUREMENT INEPIDEMIOLOGY Rates Ratios Proportions Jan 14, 2018 9
- 10. Jan 14, 201810 ANALYTICAL EPIDEMIOLOGY Testing a hypothesis *Statistical association between a disease and suspected factor *Strength of association The subject of interest is individual within the population Inference is to the population from which individuals are selected
- 11. Jan 14, 201811 They are of 2 types * Case control study * Cohort study
- 12. Jan 14, 201812 CASE- CONTROL STUDY Subjects are selected on the basis of whether they do (cases) or do not (controls) have a particular disease under study. Compared with respect to proportion having a history of an exposure or characteristic of interest.
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- 14. Jan 14, 201814 Distinct feature Both exposure and outcome (disease) have occurred before the start of study The study proceeds backwards from effect to cause, and is thus called retrospective It uses a control or comparison group to support or refute an inference
- 15. Jan 14, 201815 Basic Steps in conducting a Case Control Study Selection of cases and controls Matching Measurement of exposure Analysis and interpretation
- 16. 1. Selection ofCases and Controls Jan 14, 2018 16 Identify a suitable group of cases and a group of controls Comparability of cases and controls is essential Major issues to be considered are: ‐ selection of study groups ‐ sources of information about exposures and disease
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- 18. Selection of cases Definition of disease – Diagnostic criteria-Once the diagnostic criteria are established, they should not be altered or changed till the study is over. - Eligibility criteria- it eliminates the possibility that the long term survivors of disease were exposed to a investigated risk factor after the onset of disease. Jan 14, 2018 18
- 19. Sources of cases Hospital General Population Jan 14, 2018 19
- 20. Selection of controls *Must be free from disease under study as similar to case as possible Comparison group identified before study is done Jan 14, 2018 20
- 21. Sources of controls Hospital Control Relatives Neighborhood control General population Jan 14, 2018 21
- 22. Pancreatic cancer &Coffee Consumption Jan 14, 2018 22
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- 24. Hospital controls Jan 14,2018 24 Advantages Easily identified and readily available More aware of antecedent exposures or events, reduce potential for recall bias Likely to have been subjected to the same intangible selection factor More likely to be willing to co operate than healthy‐ individuals, minimizing bias due to non response‐
- 25. Disadvantage – Theyare ill and therefore differ from healthy individuals in a number of ways may be associated with illness and hospitalization in general. – Select from various diagnostic groups – Don’t select patient with diagnosis known to be related to risk factor of interest Jan 14, 2018 25
- 26. General Population Fromdefined geographical areas Disadvantages: ‐ more costly and time consuming ‐ difficult to contact ‐ quality of information may differ ‐ individuals refuse to participate Jan 14, 2018 26
- 27. How many controlsare needed? Ideally, 1 case : 1 control May be 2 controls per case depending upon:size of study group time cost Multiple controls provide a check on bias Jan 14, 2018 27
- 28. 2. Matching Amajor concern in conducting a case‐ control study is that cases and controls may differ in characteristics or exposures other than the one that has been targeted for study. Jan 14, 2018 28
- 29. e.g. ifmost of the cases are poor and most of the controls are affluent, we would not know whether the factor determining development of disease is exposure to the factor being studied or another characteristic associated with being poor. Jan 14, 2018 29
- 30. Matching isdefined as the process of selecting the controls so that they are similar to the cases in certain characteristics, such as age, race, sex, socioeconomic status, and occupation. If not adequately matched, could distort or confound the results. Jan 14, 2018 30
- 31. Group Matching Groupmatching (or frequency matching): – Proportion of controls with a certain characteristic is identical to the proportion of cases with the same characteristic. e.g. if 25% of the cases are married, the controls will be selected so that 25% of that group is also married. Jan 14, 2018 31
- 32. Individual Matching Individualmatching (or matched pairs). A control is selected who is similar to the case in terms of the specific variable or variables of concern. e.g., if the first case enrolled in our study is a 25 year old‐ ‐ female, we will seek a 25 year old‐ ‐ female control. Jan 14, 2018 32
- 33. What are theproblems with matching? Practical Problems with Matching: Match according to too many characteristics, it may prove difficult or impossible to identify an appropriate control. e.g., match each case for race, sex, age, marital status, number of children, zip code of residence, and occupation. Jan 14, 2018 33
- 34. 2. ConceptualProblems with Matching: – Once we have matched controls to cases according to a given characteristic, we cannot study that characteristic. For example, suppose we are interested in studying marital status as a risk factor for breast cancer. Jan 14, 2018 34
- 35. 3. Measurement ofExposure Definition and criteria for exposure Obtain by interviews, questionnaire, past records etc. Must rule out ‘bias’ Jan 14, 2018 35
- 36. 4. Analysis Tofind out Exposure rates among cases and controls Estimation of disease risk associated with exposure (Odds Ratio) Jan 14, 2018 36
- 37. EXPOSURE RATES Acase control study provides a direct estimation of the exposure rates to a suspected factor in disease and non- disease groups. Jan 14, 2018 37
- 38. Case Control StudyAnalysis CASES CONTROLS Exposed a b Non exposed c d Jan 14, 2018 38
- 39. Exposure rates *Cases = a/(a+c) * Controls = b/(b+d) Jan 14, 2018 39
- 40. Estimtion of risk “RELATIVE RISK” – defined as the ratio between the incidence of disease among exposed persons and incidence among non exposed RR = incidence among exposed / incidence among non exposed = {a/(a+b)}/{c/ (c+d} Jan 14, 2018 40
- 41. Odds ratio (Crossproduct‐ ratio) Assumptions: The disease being studied must be relatively rare. The cases must be representing of those with the disease. The controls must be representing of those without the disease. Jan 14, 2018 41
- 42. The oddsin the exposed group are compared with the odds in the unexposed group: (Odds of disease in the exposed group) / (Odds of disease in the unexposed group) Jan 14, 2018 42
- 43. Odds ofdisease in the exposed group: a/b Odds of disease in the unexposed group: c/d Odds Ratio = (a/b) / (c/d) = ad/bc Jan 14, 2018 43
- 44. Bias Systematic errorin the determination of association between the exposure and disease. The relative risk estimate may increase or decrease as a result of the bias. Jan 14, 2018 44
- 45. Bias in CaseControl Studies Bias due to confounding Memory/Recall Bias Selection Bias Interviewer’s Bias berkesonian bias Jan 14, 2018 45
- 46. Advantages of CaseControl Studies Easy to carry out. Suitable to investigate rare diseases. No risk to the subjects. Allows study of aetiological factors. Risk factors can be identified. No attrition problems. Jan 14, 2018 46
- 47. Disadvantages of caseControl Studies Problems of bias relies on past records. Selection of control group is difficult. Can't measure incidence. Not suited to evaluation of therapy or prophylaxis of disease. Jan 14, 2018 47
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- 49. WHAT IS COHORT Ancient Roman military unit, A band of warriors. Persons banded together. Group of persons with a common statistical characteristic. [Latin] E.g. age, birth date, Jan 14, 2018 49
- 50. Cohort studiesCohort studies longitudinallongitudinal ProspectivestudiesProspective studies Forward looking study IForward looking study I Incidence studyIncidence study starts with people free of diseasestarts with people free of disease assesses exposure at “baseline”assesses exposure at “baseline” assesses disease status at “follow-up”assesses disease status at “follow-up” Jan 14, 2018 50
- 51. INDICATION OF ACOHORT STUDY When there is good evidence of exposure and disease. When exposure is rare but incidence of disease is higher among exposed When follow-up is easy, cohort is stable When ample funds are available Jan 14, 2018 51
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- 54. Frame work ofCohort studiesFrame work of Cohort studies Jan 14, 2018 54
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- 56. General consideration whileselection ofGeneral consideration while selection of cohortscohorts Both the cohorts are free of the disease.Both the cohorts are free of the disease. Both the groups should equally susceptibleBoth the groups should equally susceptible to diseaseto disease Both the groups should be comparableBoth the groups should be comparable Diagnostic and eligibility criteria for theDiagnostic and eligibility criteria for the disease should be defined well in advance.disease should be defined well in advance. Jan 14, 2018 56
- 57. Elements of cohortstudy Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis Jan 14, 2018 57
- 58. Selection of studysubjects General population – Whole population in an area – A representative sample Special group of population – Select group • occupation group / professional group (Dolls study ) – Exposure groups • Person having exposure to some physical, chemical or biological agent – e.g. X-ray exposure to radiologists Jan 14, 2018 58
- 59. Obtaining data onexposure Personal interviews / mailed questionnaire Reviews of records – Dose of drug, radiation, type of surgery etc Medical examination or special test – Blood pressure, serum cholesterol Environmental survey By obtaining the data of exposure we can classify cohorts as – Exposed and non exposed and – By degree exposure we can sub classify cohorts Jan 14, 2018 59
- 60. Selection of comparisongroup Internal comparison – Only one cohort involved in study – Sub classified and internal comparison done External comparison – More than one cohort in the study for the purpose of comparison – e.g. Cohort of radiologist compared with ophthalmologists Comparison with general population rates – If no comparison group is available we can compare the rates of study cohort with general population. – Cancer rate of uranium miners with cancer in general population Jan 14, 2018 60
- 61. Follow-up To obtaindata about outcome to be determined (morbidity or death) – Mailed questionnaire, telephone calls, personal interviews – Periodic medical examination – Reviewing records – Surveillance of death records – Follow up is the most critical part of the study Some loss to follow up is inevitable due to death change of address, migration, change of occupation. Loss to follow-up is one of the draw-back of the cohort study. Jan 14, 2018 61
- 62. ANALYSIS Calculation ofincidence rates among exposed and non exposed groups Estimation of risk Jan 14, 2018 62
- 63. Incidence rates ofoutcome Jan 14, 2018 63
- 64. Incidence rate Incidenceamong exposed = a a+b Incidence among non-exposed = c c+d Jan 14, 2018 64
- 65. Estimation of risk Relative Risk incidence of disease among exposed RR = ______________________________ Incidence of disease among non-exposed a/a+b = c/c+d Jan 14, 2018 65
- 66. Attributable Risk Incidenceof disease among exposed – incidence of disease among non exposed AR = Incidence of disease among exposed a/a+b – c/c+d AR = a/a+b Jan 14, 2018 66
- 67. Find out RRand AR for above data Smoking Lung cancer Total YES NO YES 70 6930 7000 NO 3 2997 3000 73 9927 10000 Jan 14, 2018 67
- 68. Incidence oflung cancer among smokers 70/7000 = 10 per 1000 Incidence of lung cancer among non-smokers 3/3000 = 1 per thousand RR = 10 / 1 = 10 (lung cancer is 10 times more common among smokers than non smokers) Jan 14, 2018 68
- 69. AR = 10– 1 / 10 X 100 = 90 % (90% of the cases of lung cancer among smokers are attributed to their habit of smoking) Jan 14, 2018 69
- 70. Types of CohortStudy Prospective cohort study Retrospective (historical) cohort study Combination of Retrospective and Prospective cohort study. Jan 14, 2018 70
- 71. Cohort studies Strengths Wecan find out incidence rate and risk More than one disease related to single exposure can establish cause - effect good when exposure is rare minimizes selection and information bias Weaknesses losses to follow-up often requires large sample ineffective for rare diseases long time to complete expensive Ethical issues Jan 14, 2018 71