COHORT STUDY PROSPECTIVE STUDY LONGITUDINAL STUDY INCIDENCE STUDY

COHORT STUDY PROSPECTIVE STUDY, LONGITUDINAL STUDY, INCIDENCE STUDY, FORWARD LOOKING STUDY

DISTINGUISHING FEATURES • Cohorts are identified prior to the disease under investigation • The study groups, so defined are observed over a period of times to determine the frequency of disease among them • The study proceeds forward from cause to effect

Concept of cohort • A group of people who share a common characteristics or experience within a defined time period • Eg. Birth cohort • Exposure cohort • Marriage cohort

Indication for cohort studies • When there is a good evidence of association • When exposure is rare and incidence is high among exposed • When attrition is minimized • When funds are available

Framework of a cohort study • Cohort disease yes exposed a not exposed c no b d total a+b c+d

General considerations • Cohorts must be free from disease under study • Both the groups should be equally susceptible to the disease under study, or efficiently should reflect any difference in disease occurrence • Both the groups should be comparable in respect of all possible variables, which may influence the frequency of disease • The diagnostic and eligibility criteria must be defined before hand

• Both the groups are followed up • Incidence among exposed a/a+b is significantly higher than the incidence among not exposed c/c+d

Types of cohort study • Prospective cohort study – Doll and Hills prospective study on smoking and lung cancer – Framingham heart study of U. S public health services – Uranium miners and lung cancer

Population Diseased Exposed Non diseased Non Diseased (COHORT) Non Exposed Non diseased Obtaining data on exposure TIME - DIRECTION OF INQUIRY Follow-up Selection of study subjects Diseased Measure the outcome and analyze 9

Retrospective cohort study – historical cohort study (economical) • Electronic fetal monitoring and neonatal deaths • Arsenic in carcinogenisis • Uranium miners and lung cancer • Angiosarcoma of liver in those who use PVC’s

Combination of retrospective and prospective study • Court Brown and Doll in the effects of radiation ( for ankylosing spondilitis) outcome was death leukemia or aplastic anemia

Elements of a cohort study • • • Selection of study subjects Obtaining data on exposure Selection of comparison groups Follow up Analysis

1. Selection of study subjects 1. General population 2. Special groups 1. Select groups – advantage, homogeneous easily accessible eg, . Dorn in smoking and mortality 2. Exposure groups

Obtaining data on exposure 1. Cohort members – by personal interviews and mailed questionnaire (Doll and Hills) 2. Review of records 3. Medical examination or special tests 4. Environmental surveys Information about exposure should be collected 1. Whether they are exposed 2. Degree of exposure

3. Selection of comparison groups • Internal comparison – inbuilt comparison • External comparison • Comparison with general population – Difficulties of general population – Non availability of rates – Difficulties in selecting exposed and not exposed

4. Follow up • • Periodic medical examination Reviewing physician and hospital records Routine surveillance and death records Mailed questionnaire, telephone calls, periodic home visits – all three on annual basis Loss to follow up may be the reason for bias Minimum 95% follow up

5. Analysis • Incidence rates among exposed and not exposed • Estimation of risk – Relative risk – Attributable risk – Population attributable risk

Relative risk • It is the ratio of incidence of disease among exposed to incidence of disease among not exposed • Relative risk= Incidence of disease (or death) among exposed incidence of disease (or deaths) among not exposed

• It is also called as risk ratio • It is a direct measure of strength of association between the cause and effect • RR=1, no association • RR=2, indicates the disease is 2 times higher in the exposed to that of non exposed • A relative risk of 0. 25 indicates 75% reduction in the incidence rates in the exposed to that of not exposed

2. Attributable risk • It is the difference in the incidence rate among exposed to that of non exposed • “Risk difference” • AR= incidence of disease rate among exposed – incidence of disease rate among not exposed incidence rate among exposed

• AR indicates to what extent the disease under study can be attributed to the exposure • This suggests that the amount of disease that might be eliminated if the factor under study could be controlled or eliminated

3. Population attributable risk • It is the incidence of disease in the total population minus the incidence of disease among those who are not exposed to the suspected causal factor • It shows that how much the disease could be eliminated in the population by eliminating the suspected causal factor

Advantages and disadvantages of cohort study Advantages 1. Incidence can be calculated 2. Several possible outcome can be studied 3. Cohort studies provide relative risk 4. Dose response can also be calculated 5. Since the comparison groups are formed before the disease develops bias can be eliminated

Disadvantages • It involves a large population, unsuitable for uncommon diseases • It takes long time to complete by then participants have changed their classification • Administrative problems like loss of experienced staff, funds and extensive record keeping • The original cohort people may lose interest, refuse to cooperate

• Selection of comparison groups may be difficult. Those who volunteer may not be representing the population • Changes in diagnostic criteria over time • Expensive • Study may alter peoples behavior • Ethical problems • Practical problems dictate that we should concentrate on limited factors

Differences between case control and cohort studies Proceeds from effect to cause Proceeds from cause to effect Starts with the disease Starts with the people exposed to particular cause Tests whether the suspected cause occurs more frequently with the disease or without the disease Usually first approach in testing a hypothesis Tests whether the disease occurs more frequently in those exposed than in not exposed Reserved for previously formulated hypothesis

Involves fewer number of Large numbers subjects Quick results Long follow up with delayed results Suitable for rare diseases Inappropriate when the disease is rare Yields only RR (Odd’s ratio) Both RR and AR

Cannot yield information Can yield Information about diseases other than about more than one selected for the study outcome Relatively inexpensive Expensive