Observational Cohort Studies Lecture 7.1 PDF

Observational Cohort Studies Dr. Muhammad Ahmed Alshyyab Lecture 1 LEARNING OBJECTIVES: In this lecture, you will be able to demonstrate the following key massages: To define cohort study To understand the basic design of cohort study and its criteria...

Observational Cohort Studies Dr. Muhammad Ahmed Alshyyab Lecture 1 LEARNING OBJECTIVES: In this lecture, you will be able to demonstrate the following key massages: To define cohort study To understand the basic design of cohort study and its criteria To recognise the historical perspective about cohort study and its development. To describe and understand some historically important cohort studies such as Framingham Heart Study study Cohort Studies The term cohort derives from the Latin word cohors, meaning ‘‘an enclosure.’’ Cohort studies = closed populations. Cohort studies follow two groups of individuals. One group is exposure and the other is ‘‘nonexposed.’’ Study outcomes in individuals are ascertained over time, tallied, and compared in the form of incidence rates or incidence proportions. Basic Cohort Design RR or RD RR or RD Cohort studies Also called follow-up or incidence studies, begin with a group of people who are free of disease, and who are classified into subgroups according to exposure to a potential cause of disease or outcome. Variables of interest are specified and measured and the whole cohort is followed up to see how the subsequent development of new cases of the disease (or other outcome) differs between the groups with and without exposure.  Cohort criteria Cohort studies can be both prospective and retrospective. Cohort studies provide the best information about the causation of disease and the most direct measurement of the risk of developing disease. Cohort studies are major undertakings Require long periods of follow-up since disease may occur a long time after exposure. Many exposures investigated are long-term in nature and accurate information about them requires data collection over long periods. Simplest Possible Example R1  ??? R0  ??? Simplest Possible Example 2 deaths R1 = 75 person-years = 0.0267 deaths per PY 2 deaths R0 = 175 person-years = 0.0114 deaths per PY R1.0267 RR    2.33 R0.0114  Historical Perspective From the Latin cohors – “an enclosure” (also, William Farr (1807-1883) a unit of the Roman army) recognized the essence of Read about early historical developments in the cohort method: “not the text, for example… losing sight of individual – Ancient studies of workers (e.g., Ramazzini, 1713) experiences from the – Early clinical cohorts (Louis; Pinel; 18th/19th beginning to the end” century) – The British Doctors Study (Doll & Hill; 20th century) 1951 to 2001 – Framingham Heart Study (20th century) Goldberger on Pellagra Joseph Goldberger (1874–1929) demonstrated nutritional basis of pellagra Pellagra initially thought to be contagious Goldberger’s cohort study of cow ownership & pellagra: Cow Pellagra Pellagra Ownership + − Cow + 2 125 127 Cow – 47 404 451 R1=2/127 =0.0157 RR=R1/R2=0.0157/0.1042= 0.15 R2=47/451=0.1042 Observational cohort studies (Framingham Heart Study) Six-year incidence of coronary heart disease according to initial serum cholesterol level, 40–59 year old Framingham Heart Study participants.