INTRODUCTION:

• Cohort study is a type of analytical study.
• Proceeds from cause to effect(Also seen in RCT and ecological studies).
• Supports the existence of association between suspected cause and disease.
• Indicated when there is good evidence of an association between an exposure and disease.
• It is a study is one in which the outcome (e.g. disease) has not yet occurred at the time the investigation begins.
• It is also called longitudinal study, incidence study or forward-looking study.
• Most preferable observational/analytic Study design
• Least preferable observational/analytic study design-Ecological Study.
• Most reliable, overall → systematic review and meta-analysis of RCTs
• Most reliable individual study → Randomized controlled trial.

• Advantage of Case control studies over Cohort Study is that Odds ratio(estimate of Relative Risk)  can be measured and rare disease can be studied but Chances of bias are maximum in case-control study.
• Can not study many causal factors at a time as in Case Control Study.
• Cohort study along with case control study and randomized control trials are based on the comparative study of two groups.
• Matching is not required in a case of Case report.
• Natural history of a disease is best studied by cohort studies.
• Case control study yields only estimate of relative risk.
• Cross sectional study yields prevalence rate.
• Study of the group who do not have the disease of interest initially but develops with time or not. 

BIAS ASSOCIATED WITH COHORT STUDY:

• Selection
• Confounding
• Interviewer’s bias
• Attention bias (hawthorne effect)- Study subject systemically alter their behavior when they are being observed as in follows up of Cohort study
• Chances of bias are maximum in case-control study.
• Non-Bias Study is Randomised Controlled Trials.

RESULTS OF COHORT STUDY:

INCIDENCE RATE

• Incidence Rate is defined as the number of new cases occurring in a defined population during a specified period of time.
• Incidence rate = Number of cases in a defined period/mean number of persons exposed to risk in that period * 1000

• Relative risk is the ratio of incidence of disease among exposed and the incidence among non-exposed
• It is a direct measure of strength of the association between the suspected cause and effect
• Relative risk of 1 indicates no association
• Greater than one indicates positive association

ATTRIBUTABLE RISK (RISK DIFFERENCE)

• Difference in the incidence rate between an exposed group and a non-exposed group
• Suggests the amount of the disease that might be eliminated if the factor under study could be eliminated.
• Attributable risk indicates to what extent the disease under study can be attributed to the exposure.
• Example-In a cohort study nonsmokers were found to have carcinoma lung. This indicates that Etiology for lung Ca is multifactorial
  

POPULATION ATTRIBUTABLE RISK

• Incidence of disease in the total population minus among those who were not exposed
• Estimate of the amount that the disease could be reduced in the population if the suspected factor is controlled

TYPES OF COHORT STUDY:

Prospective cohort studies:

• Current cohort study or a prospective cohort study proceeds from cause to effect.
• Outcome has not yet occurred when the study has begun: Only exposure has occurred; we look for development of same disease in both exposed and non-exposed groups
• Example:Framingham Heart study, Study of OCPs and health by Royal college of General practitioners, Doll and Hills study on smoking and lung cancer.

Retrospective cohort studies: 

• Known as ‘Historical cohort study’ or Won-concurrent cohort study’
• Combines advantages of both Cohort study and Case control study
• Both exposure as well as outcome have occurred when the study has begun.
• Sample size required is same as that of prospective cohort study.
• Usually used in occupational cancer studies
• Example:Roles of Arsenic in human carcinogenesis, mortality of groups of physicians in relation to exposure to radiation, polyvinyl chloride in liver angiosarcoma.

Combined prospective-retrospective cohort study

• Known as Mixed cohort study
• Combines designs of both prospective cohort study and retrospective cohort study
• Both exposure as well as outcome have occurred when the study has begun
• Examples :– Court-Brown & Doll Study on effects of radiation therapy

ADVANTAGES:

• Incidence can be calculated
• Several possible outcomes related to exposure can be studied simultaneously.
• Study of risk factor associated with multiple diseases.
  
• Relative risk can be estimated
• Dose-response rations can be calculated 

DISADVANTAGES:

• Involves large number of people
• Unsuitable for investigating diseases with low incidence
• Long time
• Difficult to keep large number of individuals under medical surveillance indefinitely
• Administrative problems – loss of experienced staff, loss of funding, extensive record keeping is inevitable
• Substantial proportion of original cohort can be lost.
• Differential loss to follow up.
• There may be changes in the standard methods or diagnostic criteria of disease over prolonged follow-up
• Expensive
• The study itself may alter behaviour of the cohort
• Ethical problems

Don’t Forget to Solve all the previous Year Question asked on Cohort Study
Click Here to Start Quiz