Cohort Study

COHORT STUDY
  • Cohort study is a type of analytical study.
  1. Types of Study:
  2. Observational studies
  3. a Descriptive studies 
  • b Analytical studies-Ecological,Cross Sectional,Cohort and Case Control Study.
  1. Experimental studies
  2. RCT or clinical trials
  3. Field trials or community intervention
  4. Community trials.
  5. Simplest form of observational Study- Cross-sectional Study
  • Most preferable observational/analytic Study design - Cohort Study
  1. Cohort study is best to test the association between risk factor and disease.
  2. Least preferable observational/analytic study design - Ecological Study.
  • To test the association between risk factor and disease, the weakest study design is Ecological Study.
  1. Most reliable, overall —> systematic review and meta-analysis of RCTs
  2. Most reliable individual study —> Randomized controlled trial.
  • Proceeds from cause to effect(Also seen in RCT and ecological studies).
  • Cohort study is undertaken to support the existence of association between suspected cause and disease.
  • Current cohort study or a prospective cohort study proceeds from cause to effect. 
  • Cohort Studies are indicated when there is good evidence of an association between an exposure and disease.Identifies several diseases due to a single risk factor.
  • It is a study is one in which the outcome (e.g. disease) has not yet occurred at the time the investigation begins.
  1. It is also called longitudinal study, incidence study or forward-looking study.
  2. Yields incidence rate, relative risk, attributable risk(except Odds Ratio-a measure of the strength of the association between risk factor and outcome).
  • 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.
  1. Matching is not required in a case of Case report.
  2. Natural history of a disease is best studied by cohort studies.
  3. Case control study study yields only estimate of relative risk.
  4. Cross sectional study yields prevalence rate.
  5. Provide strong evidence for cause to effect relationship.
  • Randomized control trials produce best evidence for evidence of causation.
  • Crossectional studies provide weaker evidence .
  • Ecological studies provide the least satisfactory evidence for cause effect relationship.
  • Study of the group who do not have the disease of interest initially but develops with time or not.
TYPES OF STUDY
  • Observational studies
  1. Descriptive studies 
  2. Analytical studies-Ecological,Cross Sectional,Cohort and Case Control Study.
  3. Experimental studiesSimplest form of observational Study- Cross-sectional Study
  4. RCT or clinical trials
  5. Field trials or community intervention
  • Community trials.
  1. Most preferable observational/analytic Study design - Cohort Study
  2. Cohort study is best to test the association between risk factor and disease.
  3. Least preferable observational/analytic study design - Ecological Study.Most reliable, overall —> systematic review and meta-analysis of RCTs
  4. To test the association between risk factor and disease, the weakest study design is Ecological Study.
  5. Most reliable individual study —> Randomized controlled trial.
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
  • Relative risk is the ratio of incidence of disease among exposed and the incidence among non-exposed
  • Estimation of relative risk is important in etiological enquiries.
  • 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
ADVANTAGES AND DISADVANTAGES OF COHORT STUDY
  • Advantages of Cohort Study
  • 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 of cohort study
  • Involves large number of people
  • Unsuitable for investigating uncommon diseases or diseases with low incidence
  • It takes a long time to complete the study and obtain results
  • Difficult to keep a large number of individuals under medical surveillance indefinitely
  • Administrative problems - loss of experienced staff, loss of funding, extensive record keeping is inevitable
  • A substantial proportion of original cohort can be lost - they may migrate, lose interest.Selection of comparison groups which are representative of the exposed and unexposed groups Differential loss to follow up which can introduce bias is one of the major practical problem in a cohort study.
  • There may be changes in the standard methods or diagnostic criteria of the disease over prolonged follow-up
  • Expensive
  • The study itself may alter behaviour of the cohort
  • Ethical problems
TYPES OF COHORT STUDY
  • Prospective cohort studies:
  1. Known as 'Current cohort study' or 'Concurrent cohort study
  2. 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
  3. 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: 
  1. Known as 'Historical cohort study' or Won-concurrent cohort study'
  2. Combines advantages of both Cohort study and Case control studyBoth exposure as well as outcome have occurred when the study has begun: First we go back in time and take only exposure into consideration (cohorts identified from past hospital/ college records), then look for development of same disease in both exposed and non-exposed groups.
  3. Sample size required is same as that of prospective cohort study.
  4. Usually used in occupational cancer studies
  5. Example:Roles of Arsenic in human carcinogenesis, mortality of groups of physicians in relation to exposure to radiation, polyvinyl chloride in liver angiosarcoma.
  6. Combined prospective-retrospective cohort study
  • Known as Mixed cohort study'
  1. Combines designs of both prospective cohort study and retrospective cohort study
  2. Both exposure as well as outcome have occurred when the study has begun: First we go back in time and take only exposure into consideration (cohorts identified from past hospital/ college records), then look for development of same disease in both exposed and non-exposed groups; later cohort is followed prospectively into future for outcome
  3. Examples :– Court-Brown & Doll Study on effects of radiation therapy
Exam Question
  • Cohort study is a type of analytical study.
  • Cohort study is best to test the association between risk factor and disease.
  • To test the association between risk factor and disease, the weakest study design is Ecological Study.
  • Proceeds from cause to effect(Also seen in RCT and ecological studies).
  • Cohort Studies are 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.
  • Yields incidence rate, relative risk, attributable risk(except Odds Ratio-a measure of the strength of the association between risk factor and outcome).
  • 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.
  • Randomized control trials produce best evidence for evidence of causation
  • Cohort Study is a Study of the group who do not have the disease of interest initially but develops with time or not.
  • Cohort study is best to test the association between risk factor and disease.
  • To test the association between risk factor and disease, the weakest study design is Ecological Study.
  • Bias associated with Cohort's study are Selection,Confounding,Interviewer's and Attention Bias(Hawthorne Effect)
  • Relative Risk is a direct measure of strength of the association between the suspected cause and effect.
  • 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.
  • Several possible outcomes related to exposure can be studied simultaneously in a Cohort Study.
  • Cohort Study is a Study of risk factor associated with multiple diseases.
  • Cohort Study involves large number of people and is expensive.
  • Cohort Study is unsuitable for investigating uncommon diseases or diseases with low incidence.
  • Cohort Study takes a long time to complete the study and obtain results
  • Differential loss to follow up which can introduce bias is one of the major practical problem in a cohort study.
  • Outcome has not yet occurred when the study has begun: Only exposure has occurred-It is called Prospective or Concurrent or Current Cohort Study.
  • Framingham Heart study is a type of Cohort Study.
Don't Forget to Solve all the previous Year Question asked on Cohort Study