COHORT STUDYByDr.N.Bayapa ReddyPost GraduateDept.of Community MedicineS.V.Medical collegeTirupati : COHORT STUDYByDr.N.Bayapa ReddyPost GraduateDept.of Community MedicineS.V.Medical collegeTirupati COHORT STUDY : COHORT STUDY PROSPECTIVE STUDY
FORWARD LOOKING STUDY
PANEL STUDY Concept of cohort : Concept of cohort It is an observational analytical study
Cohorts are “a group of people who share common characteristics or an experience within a defined time period”.
Ex: Birth cohort
Marriage cohort DISTINGUISHING FEATURES : DISTINGUISHING FEATURES It attempts to study the relationship between a purported cause (exposure) and the subsequent risk of developing the disease.
There is neither randomization nor attempt to manipulate the exposure.
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. Indications for cohort studies : Indications for cohort studies When there is a good evidence of association.
When exposure is rare and incidence is high among exposed.
When attrition is minimum.
When ample funds are available. Framework of a cohort study : Framework of a cohort study Cohort Disease
yes no Total
Exposed a b a+b
Not Exposed c d c+d
Total a+c b+d General considerations : 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. General considerations Contd. : General considerations Contd. 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 : Types of cohort study Prospective cohort study/current
Doll and Hills prospective study on smoking and lung cancer
Framingham Heart Study of U.S Public Health Services.
Uranium miners and lung cancer. Retrospective cohort study Historical cohort study (Economical) : Retrospective cohort study Historical cohort study (Economical) Electronic fetal monitoring and neonatal deaths.
Arsenic in carcinogenesis.
Uranium miners and lung cancer
Angiosarcoma of liver in those who use PVC’s Combination of retrospective and prospective study : Combination of retrospective and prospective study Court Brown and Doll in the effects of radiation ( for ankylosing spondilitis) outcome was death or aplastic anemia Elements of a cohort study : Elements of a cohort study Selection of study subjects
Obtaining data on exposure
Selection of comparison groups
Analysis 1. Selection of study subjects : 1. Selection of study subjects General population
Select groups – advantage, homogeneous easily accessible eg,. Dorn in smoking and mortality
Exposure groups Obtaining data on exposure : Obtaining data on exposure Cohort members – by personal interviews and mailed questionnaire (Doll and Hills)
Review of records
Medical examination or special tests
Information about exposure should be collected
Whether they are exposed
Degree of exposure 3. Selection of comparison groups : 3. Selection of comparison groups Internal comparison – inbuilt comparison
Comparison with general population
Difficulties of general population
Non availability of rates
Difficulties in selecting exposed and not exposed 4. Follow up : 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 : 5. Analysis Incidence rates among exposed and not exposed
Estimation of risk
Population attributable risk Relative Risk (RR) : Relative Risk (RR) It is the “ratio of incidence of disease among exposed to incidence of disease among non- exposed”.
Incidence among exposed
Relative Risk = ----------------------------------------------------- Incidence among not exposed Relative Risk Contd. : Relative Risk Contd. 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- the disease is 2 times higher in the exposed to that of non exposed
RR= 0.25 - 75% reduction in the incidence rates in the exposed to that of not exposed Attributable Risk (AR) : Attributable Risk (AR) It is the “difference in the incidence rate among exposed to that of non exposed”.
Also known as “Risk difference”.
incidence among exposed – incidence among non-exposed
AR = -------------------------------------------------
incidence rate among exposed ATTRIBUTABLE RISK Contd. : ATTRIBUTABLE RISK Contd. 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. Population Attributable Risk : 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 disease could be eliminated in the population by eliminating the suspected causal factor. Advantages of cohort study : Advantages of cohort study Incidence can be calculated
Cohort studies offer the possibility of studying the full range of effects of the suspected etiological factor like natural H/o disease and other possible related health conditions which may not be suspected at the beginning of the study.
Several possible outcomes can be studied
Cohort studies provide relative risk
Dose response can also be calculated
Since the comparison groups are formed before the disease develops bias can be eliminated
cohort analysis is that the study design does not require strict random assignment of subjects Disadvantages of cohort study : Disadvantages of cohort study Selection of comparison groups may be difficult. Those who volunteer may not be representing the population
Changes in diagnostic criteria over time
Study may alter peoples behavior
Practical problems dictate that we should concentrate on limited factors
Ethical problems 7.Selection of comparison groups may be difficult. Those who volunteer may not be representing the population
8. Changes in diagnostic criteria over time
10.Study may alter peoples behavior
12.Practical problems dictate that we should concentrate on limited factors Differences between case control and cohort studies : Differences between case control and cohort studies Differences Contd. : Differences Contd. Differences Contd. : Differences Contd. Slide 29: TANQ