Comunicación y Gerencia 22/12/20101Cohort studies

Cohort Studies Dr. Salwa Tayel Associate Professor Family & Community Medicine Department King Saud University 22/12/20102Cohort studies

Objectives 1.To understand the basic design features of cohort studies 2. To list the advantages and disadvantages of case control studies 3.To calculate measure of association in cohort studies 22/12/20103Cohort studies

Types of Study Designs DesignStudy Type Case reportObservational - Descriptive Case seriesObservational - Descriptive Cross sectionalObservational - Descriptive Case controlObservational - Analytic CohortObservational - Analytic Clinical trialExperimental - Analytic 422/12/2010Cohort studies

Basic Question in Analytic Epidemiology Are exposure and disease linked? Direction of inquiry in cohort study Exposure Risks e.g Tobacco chewing Disease e.g. Myocardial Infarction (MI) 22/12/20105Cohort studies

Analytic Epidemiologic Study Designs 2- Cohort Studies an “observational” design where the investigator categorizes individuals on the basis of exposure thus comparing individuals with a known risk factor or exposure with others without the risk factor or exposure an “observational” design where the investigator categorizes individuals on the basis of exposure thus comparing individuals with a known risk factor or exposure with others without the risk factor or exposure looking for a difference in the risk (incidence) of a disease over time. looking for a difference in the risk (incidence) of a disease over time. 22/12/20106Cohort studies

What is a cohort?  Cohort is a group having a common characteristic Example: A smoker’s cohort means all are smokers in that group  A cohort, which is exposed to a suspected factor but not yet developed the disease, is observed and followed over time.  Then, the incidence of the disease is measured directly. 22/12/20107Cohort studies

Key Basis for selection of group for study = present= absent Prospective Cohort Study ? ? ExposureDisease 22/12/20108Cohort studies

Cohort Design time Study begins here Study population free of disease Exposure present Exposure Not present disease no disease disease no disease present future 22/12/20109Cohort studies

Cohort Study (Prospective) Exposed Unexposed Disease occurs No disease Disease occurs Future2010 Present 22/12/201010Cohort studies

It is the best observational design. Why?  The investigator proceeds from “E to D” i.e. from cause to effect so he will not face a chicken egg dilemma and the temporal (time) sequence between E and D can be clearly established.  It uses a control group to accept or reject the hypothesis between E and D. 22/12/201011Cohort studies

Issues in analysis: The basic analysis involves:  Calculation of incidence rates among the exposed = (a/a+b)  Calculation of i ncidence rates among the non-exposed = (c/c+d ) 22/12/201012Cohort studies

Frame work of a cohort TotalOutcome Not diseased Diseased a + b c + d bdacExposedNon-exposed (a/a+b) (c/c+d ) Incidence 22/12/201013Cohort studies

Quantify risk: RR is measure of association between incidence of disease and certain exposure Relative risk, Risk Ratio (RR) answers the question: “How many times a person who is exposed to risk factor is at risk of developing disease compared to non- exposed?” 22/12/201014Cohort studies

a / (a + b) c / (c + d) = Relative Risk Disease Exposure YesNoTotal Yes ab a + b No cd c + d Total a + c b + c a + b + c + d 22/12/201015Cohort studies

Interpretation of Relative Risk (RR) No association RR=1: No association between exposure and disease incidence rates are identical between groups increased risk RR> 1: Positive association ( increased risk ) exposed group has higher incidence than non- exposed group protective effect RR< 1: Negative association ( protective effect ) non-exposed group has higher incidence 22/12/201016Cohort studies

Example: Incidence in smokers = 84/3000 = 28.0/1000 Incidence in non-smokers = 87/5000 = 17.4/1000 Relative risk = 28.0/17.4 = /12/201017Cohort studies

1.Valuable in rare exposures. 2.Can study multiple effects of a single exposure. 3.Exposure happened before outcome (time relation is established) (Temporality) 4.Can calculate incidence rates. 5.Can quantify Risk, Relative risk, & Attributable Risk 6.Dose response ratio can be calculated. 7.Low potential for bias than case-control study 22/12/201018Cohort studies

1.Attrition (loss to follow up) may affect validity of results. 2.Measurement errors, multiple interviews, tests 3.Involve a large sample 4.Inefficient for evaluation of rare diseases. 5.Takes a long time. 6.Expensive. 22/12/201019Cohort studies

20 Thank You Website 22/12/2010Cohort studies