Download to read offline
Cohort study in medical research methodology
- 1. Major (Dr) RaghavendraRH Assistant Professor KSHEMA
- 2.
- 3. Cohort study Cohort =Group of people who share a common characteristic or experience within a defined time period (age, occupation ,exposure etc.).
- 4. Cohort study • Cohortstudy: are observational studies in which the investigator determines the exposure status of subjects and then follows them for subsequent outcomes – Forward looking study – Incidence study – Longitudinal study – Prospective study – Follow up study
- 5.
- 6. General Considerations • Cohortsmust be free from the disease under study. • Study and control group must be easily susceptible to the disease under study • Both the groups must be comparable in respect to all the possible variables which may influence the frequency of the disease.
- 7. General Considerations • Thediagnostic and eligibility criteria of the disease must be defined before hand. • Groups are then followed, under the same identical conditions, over a period of time to determine the outcome of the exposure.
- 8. INDICATIONS FOR COHORT STUDY •When there is good evidence of an association between exposure and disease, supported by descriptive and case control studies. • When exposure is rare, but the incidence of the disease if high among the exposed. • When there is no/minimal attrition problem. • When ample funds are available.
- 9. Elements of Cohortstudy • Selection of study subject • Obtaining the data on the exposure. • Selection of the comparison group. • Follow up • Analysis
- 10. Elements of Cohortstudy 1. SELECTION OF STUDY SUBJECTS: • The general population – when exposure or cause of illness is frequent in the population. • Special groups like select groups or exposure groups
- 11. Elements of Cohortstudy 2. OBTAINING DATA ON EXPOSURE: Information about exposure may be obtained through – Survey – Interviews – Questionnaires – Medical records
- 12. Elements of Cohortstudy 3. SELECTION OF COMPARISON GROUP: - – Internal comparisons – External comparisons – Comparison with general population rates
- 13. Elements of Cohortstudy 4. FOLLOW UP: Regular follow up is required which comprises of – Periodic medical examination, – Reviewing hospital records, – Surveillance of death records, – Mailed questionnaires, – Telephone calls, – Periodic home visits
- 14. Elements of Cohortstudy 5. ANALYSIS: • The data analyzed in terms of – Incidence rates of outcome among exposed and non exposed – Estimation of risk • Relative risk • Attributable risk
- 15. ODDS RATIO Cohort studyCase control study
- 16. Incidence rates ofoutcome N d c b a Yes No Disease Status Yes No Exposure Status a+b c+d b+d a+c Total Study cohort Comparison cohort
- 17. Incidence rate • Incidenceamong exposed = a a+b • Incidence among non-exposed = c c+d
- 18. Estimation of risk •Relative Risk RR = Incidence of disease among exposed Incidence of disease among non-exposed a/a+b c/c+d =
- 19. Interpretation of RelativeRisk and Rate Ratio • RR=1 = No association between exposure and disease – incidence rates are identical between groups • RR > 1 = Positive association – exposed group has higher incidence than non-exposed group • RR < 1 = Negative association or protective effect.
- 20. Estimation of Risk •Attributable Risk Incidence of disease among exposed – incidence of disease among non exposed Incidence of disease among exposed AR = a/a+b – c/c+d a/a+b AR% is that it tells us the quantum of reduction in the disease that would be achieved in the “exposed” group if “exposure” was given up by them.
- 21. Smoking Lung cancerTotal YES NO YES 70 6930 7000 NO 3 2997 3000 73 9927 10000 Find out RR and AR for above data
- 22. • Incidence oflung cancer among smokers 70/7000 = 10 per 1000 • Incidence of lung cancer among non-smokers 3/3000 = 1 per thousand RR = 10 / 1 = 10 (lung cancer is 10 times more common among smokers than non smokers) AR = 10 – 1 / 10 X 100 = 90 % (90% of the cases of lung cancer among smokers are attributed to their habit of smoking)
- 23. TYPES OF COHORTSTUDIES o Prospective cohort study o Retrospective cohort study o Combination of prospective and retrospective study
- 24. Types of cohortstudies Define population Variable under study Exposed Non exposed Diseased Not diseased Diseased Not diseased 2012 2014 2020 1985 2000 2014 RETROSPECTIVE PROSPECTIVE COMBINED 1985 2014 2020
- 25. FRAMINGHAM HEART STUDY •Best-known cohort studies is the Framingham Study of cardiovascular disease. • Started in 1948. • Residents between 30 and 62 years of age were considered eligible for study. • 1971 enrolled a second generation of participants. • In April 2002, a third generation was enrolled in the core study.
- 26. FRAMINGHAM HEART STUDY Results: •1960s: Cigarette smoking Increased cholesterol and elevated blood pressure obesity increases risk of heart disease. Exercise decreases risk of heart disease. • 1970s: Elevated blood pressure increases risk of stroke. Postmenopausal women risk of heart disease is increased compared with who are premenopausal. • 1980s High levels of HDL cholesterol reduce risk of heart disease. • 1990s: Elevated blood pressure can progress to heart failure. At 40 years of age, the lifetime risk for CHD is 50% for men and 33% for women.
- 27. FRAMINGHAM HEART STUDY •2000s “High normal blood pressure" increases risk of cardiovascular disease (high normal blood pressure is called prehypertension in medicine; • Defined as a systolic pressure of 120–139 mm Hg and/or a diastolic pressure of 80–89 mm Hg). • Lifetime risk of developing elevated blood pressure is 90%. Serum aldosterone levels predict risk of elevated blood pressure. • Lifetime risk for obesity is approximately 50%.
- 28. FRAMINGHAM HEART STUDY •Hypothesis: Incidence of CHD increases with age Hypertension increases CHD Elevated cholesterol is associated with increased CHD Tobacco smoking and habitual use of alcohol increased CHD Increased physical activity a/w with decreased incidence of CHD Increased Body weight inceases incidence of CHD Diabetes increases incidence of CHD
- 29. Cohort studies Strengths • Wecan find out incidence rate and risk • More than one disease related to single exposure • Can establish cause - effect • Good when exposure is rare • Minimizes selection and information bias Weaknesses • Losses to follow-up • Often requires large sample • Ineffective for rare diseases • Long time to complete • Expensive • Ethical issues
- 30.
- 31.
- 32.