COHORT STUDY COHORT A group of people who share a common characteristic or experience within a defined period of time. e.g. age, occupation, exposure to drug, exposure to a vaccine, exposed to an infection, insured persons, smokers, obese, pregnant women, babies born on a single day or in a year.
COHORT STUDY A group of individuals that is exposed to a risk factor (study factor) is compared with a group of individuals not exposed to a risk factor (control group).
Cohort study Cohort study usually proceeds from cause to effect. Exposure has occurred but the disease has not. In Cohort Study: Study group is identified prior to appearance of disease. Control group is identified prior to the appearance of disease. Study groups are observed over a period of time to determine the frequency of disease among them.
Cohort study
Cohort study Cohort must be free from disease under study Both study and control groups must be equally susceptible to disease under study Both the groups should be comparable in respect to all possible variables Diagnostic and eligibility criteria of the disease must be defined beforehand. Groups must be followed under same identical conditions over a period of time to determine outcome of exposure.
Cohort study
Cohort study After end of follow up We can calculate Incidence rate among exposed
Cohort study After end of follow up We can calculate Incidence rate among exposed = a / a + c
Cohort study After end of follow up We can calculate Incidence rate among exposed = a / a + c Incidence rate among non exposed =c / c + d
Cohort study After end of follow up We can calculate Incidence rate among exposed = a / a + c Incidence rate among non exposed =c / c + d Cohort study is most reliable mean of showing an association between a suspected risk factor and subsequent disease.
Cohort study Elements of cohort study Selection of study subject Obtaining data on exposure Selection of comparison group Follow up Analysis
STEPS IN CCS Selection of cases and controls Matching Measurement of exposure and Analysis and interpretation
1.Selection of study subjects General population Special group Select groups------homogeneous group Exposure group---experience of exposure Physical Chemical Disease agent
2. Obtaining Data On Exposure Cohort members Personal interviews Mailed questionnaire Review of records Medical examination or special tests Environmental surveys Information required for: Age, sex, gender & demographic variables etc.
3. Selection of comparison group Internal comparisons External comparisons Comparison with general population rates
4. Follow up Regular follow up of all participants Methods should be devised to measure outcome Procedures required are; Periodic medical examination of each member Reviewing of physician, medical & hospital record Routine surveillance of death records Mailed questionnaire, telephone calls, periodic home visits etc.
5.Analysis Incidence rates of outcome Among exposed Among non-exposed Estimation of risk Relative risk Attributable risk
CIGARETTE SMOKING AND LUNG CANCER
1.Incidence rates Among exposed = a/ a+b=70/7000 = 10 per 1000
1.Incidence rates Among exposed = a/ a + b = 70 / 7000 = 10 per 1000 Among non exposed= c/ c + d =3 / 3000 = 1 per 1000 P<0.001
Cohort study (CHD)
Cohort study (HTN)
2.ESTIMATION OF RISK Relative risk incidence among exposed RR= _________________________ incidence among non exposed RR=10/1=10(smokers are 10 times greater risk of developing Ca Lung) It is a direct measure of strength of association between suspected cause and effect. RR=1 no association RR=>1 positive association b/w expos & dis.
ATTRIBUTABLE RISK Is the difference in incidence rates of disease between an exposed group and non exposed group. It is expressed as percent. incidence of disease rate among AR=exposed—incidence of disease rate among non exposed incidence rate among exposed AR=10-1/10X100=90% (90% of lung cancer was due to lung cancer).
Advantages of Cohort study Incidence can be calculated Several possible outcomes related to exposure can be studied Direct estimate of relative risk Dose response can be calculated Minimized bias
Disadvantages of cohort study Large no. of people are involved Takes long time to complete the study Administrative problems, loss of staff Loss of original cohort Selection of comparison groups Change in standard methods Expensive Change in people behaviour
Differences between case control and cohort studies
