Veterinary Epidemiology and Zoonosis

COHORT STUDY

• Cohort study is also referred to as follow-up study, prospective study and incidence.
• Cohort study means looking backwards.
• A group exposed to factors is compared with a group not exposed to factors with respect to the development of a disease.
• It is then possible to calculate the level of risk of developing disease in relation to exposure to the hypothesized causal factor.
• It is then possible to calculate the incidence rate.
• In this study, characteristics or factors used to define a population are called maneuver, those possessing them are called cohorts (those who share a common characteristic).
• This study is usually undertaken to either accept or reject a hypothesis. Here the hypothesis formulated by the cross- sectional or the case-control study is tested as additional evidence.
• The important features of this study are,
  • The cohorts are identified prior to the appearance of disease under investigation.
  • The study group i.e. the case cohorts and control cohorts are observed over a period of time to determine the frequency of a disease among them. 
  • The study proceeds forward from cause to effect.

Indications for cohort study

• When there is good evidence of an association between exposure and the disease, this is evidenced by cross sectional and case control studies.
• When exposure is rare and incidence is high among exposed.
• When ample funds are available.

The following facts should be considered while selecting cohorts

• The cohort must be free from disease when the study is undertaken.
• Both groups should be susceptible to the disease.
  • Example: Persons of 35 and above years of age should be selected for lung cancer study.
• Both groups should be comparable.
• The diagnostic criteria to be used must be well defined.

Types of cohort study

• Prospective study (most commonly used)
• Retrospective study
• Combination of both

Elements of cohort study

• Selection of study subjects
• Obtaining data on exposure
• Selection of comparison groups
• Follw up
• Analysis

Advantages

• No biased information.
• Incidence rate can be calculated.
• Other associated problems can be studied simultaneously.
• It provides direct estimate of relative risk.
• Dose-response relationship can also be calculated.

Disadvantages

• Long, expensive and large-scale undertaking.
• Cohorts may vary. So should be properly designed.
• Administrative problems with long-term study.
• Follow up is difficult.
• Exposed and unexposed proportions in target population cannot be estimated.
• Large number of subjects are required to study rare diseases.