2 Cohort :- group of people who share a common characteristic or experiences within a defined time period (e.g.. Age, Occupation, Exposure to drugs or vaccine, Pregnancy, insured persons etc.) All those borne in 1980 can be regarded as a birth cohort of 1980.
3 Some features…Comparing 2 or more groups forwards from exposure to outcomes.Prospective (ahead in time) or retrospective (Back in time).Examines outcomes after single exposure.Less useful for rare diseases.Best way to identify incidence and natural history.
8 The Basic Approach of the Cohort…….. Proceeds from cause to effect.
9 Cohort studyForward-looking study, prospective study or longitudinal study or incidence studyGroup of individuals are defined on the basis of their exposure status and are followed to assess the occurrence of diseaseCohort study is designed to answer the question– what are the effects of this particular exposure?
10 Features of cohort study The investigation starts with a group of individuals at risk who are free of disease of interest.The study groups are divided into those exposed to a risk factor and those not exposed, and then followed over a period of time to determine the frequency of disease.The study proceeds forward from cause to effects.
11 PRESENT FUTURE (Prospective by collection of data looking forward) Exposed to risk factorNot exposed to risk factorProblem presentProblem absentCompare
12 CohortCohort is defined as a group of people who share a common characteristics or experiences within a defined time period.A group of people born on the same day or year -birth cohortPerson exposed to a common drugs, vaccines- exposure cohortA group of male and female married on the same day or in the same period of time - marriage cohortThe cohort selected should have following characteristicsThe cohort must be free from the disease under study.Study and control cohorts should be equally susceptible to the disease under study.Both the groups should be comparable.The diagnostic and eligibility criteria of disease should be defined before hand.
13 Steps in cohort studies Selection of cohortDetermining exposure statusSelection of comparison groupsFollow up or measurement of diseaseAnalysis
14 Selection of cohortPopulation at risk should be identified in order to select the susceptible persons.The cohort can be selected either from general population or from special group.Special group -group of doctors, teachers, injured personIdentification of persons with current or past asymptomatic or subclinical disease sometimes presents difficultiesUse of laboratory tests or other special procedures may help to identify persons who should be excluded from the studyThe procedure of the investigation must be simple, harmless and inexpensive
15 Determining exposure status Information on exposure may be obtained from:Cohort members through personal interviews or mailed questionnaires (age, smoking habits etc.)Review of records from medical recordsMedical examination or special test (blood pressure, serum cholesterol, ECG etc.)Environmental surveysThe investigator makes the measurements and then classifies the cohort members into various exposure categories.
16 Selection of comparison groups Internal comparison Among the members of cohort.On the basis of information obtained, they are classified into several comparison groups according to the degree or levels of exposure to risk.External comparisonWhen information and degree of exposure is not available, external control is set up and compared.Comparison with general populationIf above data is not available, the exposed group are compared with general population.
17 Follow up or measurement of disease The problem in cohort study is regular follow up of all participants.Periodic medical examination of each member of cohortReviewing physician and hospital recordsRoutine surveillance of death records, vital recordsMailed questionnaires, telephone calls, periodic home visitsEnvironmental measuresDuring follow-up, some members may be lost due to death, change of residence, migration withdrawal of occupations
18 a. Incidence rate among exposed = a/a+b AnalysisNo. ofexposedunexposedTotalDiseaseaba +bNo diseasecdc +da +cb +dNa. Incidence rate among exposed = a/a+bb. Incidence rate among unexposed = c/c+d
19 Relative riskRatio of the cumulative incidence among exposed to the cumulative incidence among unexposed.It provides the understanding the effect of an exposure knowing the frequency of disease in the absence of exposure (baseline or background rate).It indicates the likelihood of developing the disease in the exposed group relative to those who are not exposed.Interpretation: The risk of developing disease is n times higher among exposed compared to unexposed persons in this population during time t.RR > 1, Risk or rate of new disease is greater in exposed group than in unexposed group during time t; direct relationship between exposure and outcome.RR = 1, Risk or rate of new disease are identical in both groups during time t; no relationship between exposure and outcome.RR < 1, Risk or rate of new disease is less in exposed group than in unexposed group during time t; protective or inverse relationship between exposure and outcome.
20 Strengths of cohort studies Cumulative incidence and incidence density can be calculated.One can assess the multiple effects of a particular exposure.Temporal relationship between exposure and outcome can be established.Relative risk, dose response ratio can be calculated.Bias can be minimized.New or unusual exposure can be studied.
21 Weaknesses of cohort studies It involves large number of people. This is unsuitable for uncommon disease. Rare disease can not be studied.It takes long time to complete the study.There may be administrative problems. The study may be affected by secular trends in technology.It is difficult to follow up.It is expensive.The study may change behavior of cohort.
22 Differences between case control and cohort study Case control studyCohort study1. Case control studies are cheap.1. Cohort studies are expensive.2. Case control studies tend to rely on recall for exposure measure.2. Cohort studies can measure exposure precisely.3. They do not allow for measurement of disease prevalence.3. Disease prevalence can be measured.4. These are efficient in study of rare disease.4. These are impractical for study of rare disease.
23 Nested Case Control Study A case-control study design in which cases and controls are from the subjects in an ongoing cohort study.This is hybrids of retrospective and prospective analyses because they start by identifying a population, then collect baseline data and follow the population for the next several years.An efficient method for studying case-control relationships among a cohort once a number of cases have emerged.
24 Nested Case control Study Design Obtain interviews, bloods, urines, etc.TIME 1YEARSTIME 2Study PopulationDo NotDevelopDiseaseDevelopDiseaseCASES CONTROLSCASE-CONTROL STUDY
28 Advantages of Nested Case Control study Possibility of recall bias is eliminated, since data on exposure are obtained before disease develops.Exposure data are more likely to represent the pre-illness state since they are obtained years before clinical illness is diagnosed.Costs are reduced compared to those of a prospective study, since laboratory tests need to be done only on specimens from subjects who are later chosen as cases or as controls.
29 Limitation of Nested Case Control study The drawback of nested case-control studies is non-diseased persons from whom the controls are selected may not be fully representative of the original cohort, due to death or failure to follow-up cases.