Cohort Studies Lecture 7 - PDF

Types of Epidemiologic Studies (COHORT STUDIES) Community Medicine & Epidemiology Dr. Sanabel Barakat Year 3 – semester 1 BDs., MSc., PhD., JDC. Week 5  ILOs Learn about cohort studies Discuss different types of cohor...

Types of Epidemiologic Studies (COHORT STUDIES) Community Medicine & Epidemiology Dr. Sanabel Barakat Year 3 – semester 1 BDs., MSc., PhD., JDC. Week 5  ILOs Learn about cohort studies Discuss different types of cohort studies Understand the difference between cohort and case control studies 2  Classification of Epidemiological studies Epidemiological studies classification According to objective : Descriptive (case report/series, cross-sectional (survey), ecological, time series studies) Analytical (case-control & cohort studies,) According to methodology: Observational Descriptive (case report/series, cross-sectional (survey), ecological, time series studies) Analytical (case-control & cohort studies) Experimental (RCTs, Field trial, community trial) 3  Cohort study Cohort studies ask the question: What are the effects of this exposure? 4  Cohort studies Definition: Cohort studies compare exposed and exposure-free controls for their disease status after following them over time and compare the risk of disease in cases and controls. They can be designed as “prospective” or “retrospective studies.” Exposed and unexposed are similar in all other respects and are comparable. 6  Observational, Analytical Cohort studies A form of longitudinal observational The central objective is to assess the incidence of a disease or health condition. 7  Observational, Analytical Cohort studies It includes the perspective of comparing incidence levels between groups with different status regarding exposure factors of interest. 8  Observational, Analytical Cohort studies Select two disease free cohorts: Exposed (index cohort) Unexposed (Reference group) Then follow these study cohorts over time to see how many disease cases develop in each group. The number of new cases and the rate at which cases develop are compared between the index and reference cohorts to yield the relative risk of disease given the exposure of interest. 9  Cohort studies Cohort studies ensure that exposures occurred be fore disease outcomes. The term inception cohort is used to identify a group of persons who are aggregated together close to the onset of the disease (inception of disease). Assumptions: Disease prevalence is not low. Rare exposures 10  Cohort studies Comparison Groups: Internal comparison: A cohort of people may be followed, and the unexposed subsection of the cohort serves as the comparison group. General living population: Comparison cohort from another population may be used; available data on disease occurrence may be used. 11  Cohort studies Exposure definition and measurement is critical to the study and it must be carefully defined and measured. Clear Case definition a priori, because with new research information and understanding of disease, working definitions may change over time. Periodic follow-up should be planned in a way that new disease could be identified before its remission. 12  Cohort studies Advantages: Allows causal interpretation Can study multiple outcomes of an exposure Provides the real measure of risk of disease/relative risk & excess risk Can ascertain disease incidence (cumulative incidence and incidence density) Can incorporate information about changing patterns of risk factors 13  Cohort studies Advantages: Can assess dose–response relationship Can examine multiple outcomes of the same exposure Allows more control over subject selection, measurement, and control of measurement bias Multiple cohorts can be studied Smaller case-control studies can be nested within cohort studies to make studies more resource efficient. 14  Cohort studies Disadvantages ▪ Requires large sample size ▪ Expensive (retrospective cohort can be less expensive and logistically simpler, with shorter duration) ▪ Takes long time for completion ▪ Difficult to conduct for rare diseases ▪ Logistics ▪ Subject to missing/loss of participants data due to follow-up bias 15  Cohort studies Disadvantages ▪ Subject to healthy worker effect ▪ “Case definition” may change in the future with new research ▪ New diagnostic techniques may come out and compromise the study ▪ May be affected by secular trends ▪ If multiple cohorts are studied, then probability of selection bias increases. 16  Cohort studies Indications When To Do Cohort Study: When disease prevalence is high, and exposure is rare when causal association is being tested, when true relative risk is sought, when disease prevention methods are being tested, when disease does not have a very long induction period/latency, when etiological mechanisms are being assessed, When multiple effects of exposures are being examined. 17  Cohort studies Types Prospective Retrospective 19  Prospective cohort In cohort studies, the starting point is the exposure. The participants are healthy at the beginning and are prospectively monitored over time to record the outcome. At a given moment, the incidence of the disease is measured in the exposed and non-exposed, allowing the calculation of the relative risk. This form of organizing the data implies that, in general, cohort studies are prospective. 20 PRESENT 21  Retrospective cohort For the configuration of retrospective cohorts, It is PRESENT possible to consolidate data a posteriori This highlights that there are some exceptions to the general rule that cohort studies are always prospective. 22  Relative Risk (Risk Ratio) The relative risk is a measure of association used to. It is a ratio of two probabilities, RR tells how much more likely it is for people exposed to a risk/protection factor to develop the disease compared to people who are not exposed. It is commonly used in prospective studies (cohort studies) 23  You can calculate relative risk as follows: 24  Example Exposure No exposure Total Tobacco No tobacco RR= 100/150 ÷ 20/190 chewing chew =6.33 Disease a) 100 b) 20 120 Oral cancer No disease c) 50 d) 170 220 No oral cancer Total 150 190 340 25 27  OBSERVATIONAL STUDIES Retrospective Cohort 28  Thank you 29