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 Risk factor Disease yes no total Positive a b a+b Negative c d c+d

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 Risk factor Disease yes no total Positive a b a+b Negative c d c+d

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 + b 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 Estimation of risk Among exposed Among non-exposed Estimation of risk Relative risk Attributable risk

CIGARETTE SMOKING AND LUNG CANCER Developed Lung cancer Did not develop Lung cancer Total Yes No 70 (a) 3 ( c) 6930 (b) 2997 (d) 7000 (a+b) 3000 (c+d)

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) Risk factor Disease yes no total Bankers a 60 b 40 100 Non bankers c 30 d 70 c+d

Cohort study (HTN) Risk factor Disease yes no Total obese a 100 b 900 1000 Non obese c 33 d 967 c+d

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 smoking).

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 Case control study Cohort study 1. Proceeds from “effect to cause Proceeds from “cause to effect” 2. Starts with the disease Starts with people exposed to risk factor or suspected cause. 3. Tests whether the suspected cause occurs more frequently in those with the disease than among those without the disease. Tests whether disease occurs more frequently in exposed, than in those not similarly exposed. 4. Usually the first approach to the testing of a hypotheses, but also useful for exploratory studies Reserved for testing or precisely formulated hypothesis 5. Involves fewer number of subjects Involves larger number of subjects 6. Yields relatively quick results Long follow-up period 7. Suitable for the study of rare diseases Inappropriate when the disease or exposure under investigation is rare. 8. Generally yields only estimate of RR (odds ratio) Yields incidence rates. RR as well as AR. 9. Cannot yield information about diseases other than that selected for study Can yield information about more than one disease outcome. 10. Relatively inexpensive Expensive