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Cohort Study Designs

Cohort Study Designs. Ahmed Mandil Dept of Family & Community Medicine College of Medicine King Saud University. Headlines. Definitions Observational studies Cohort studies Advantages, disadvantages Examples Analysis. Observation Methods. Selected Units : individuals, groups

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Cohort Study Designs

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  1. Cohort Study Designs Ahmed Mandil Dept of Family & Community Medicine College of Medicine King Saud University

  2. Headlines • Definitions • Observational studies • Cohort studies • Advantages, disadvantages • Examples • Analysis Cohort Studies

  3. Observation Methods • Selected Units: individuals, groups • Study Populations: cross-sectional, longitudinal • Data collection timing: prospectively, retrospectively, combination • Data collection types: primary, secondary Cohort Studies

  4. Study populations • Cross-sectional: where only ONE set of observations is collected for every unit in the study, at a certain point in time, disregarding the length of time of the study as a whole • Longitudinal: where TWO or MORE sets of observations are collected for every unit in the study, i.e. follow-up is involved in order to allow monitoring of a certain population (cohort) over a specified period of time. Such populations are AT RISK (disease-free) at the start of the study. Cohort Studies

  5. Study Designs in Health Research • Qualitative • Focus group discussions • Key informant studies • Ethnographic studies • Bibliographic studies • Others • Quantitative • Observational • Descriptive, e.g. cross-sectional • Analytical, e.g. case-control; cohort • Experimental (also analytical) • Randomized Clinical Trials • Community Intervention Studies Cohort Studies

  6. Cohort Studies (I) (a) Characteristics: • A “cohort” is a group of people, referred to as “disease-free population” or “population at risk” • A survey is first carried out to exclude prevalent cases from the cohort • A period of "follow-up“ is specified, for possible new cases' occurrence • We know the exposure status, looking for the disease status • Historical designs are preferred under occupational settings, for less frequent effects / exposures. Cohort Studies

  7. Cohort Studies (II) Two types are recognized: • Prospective(longitudinal): forward in time follow-up study • Retrospective(historical): backward in time study (depends on records: medical / employment). This is the type preferred under occupational settings Cohort Studies

  8. Cohort Design disease Factor present no disease Study population free of disease disease Factor absent no disease present future time Study begins here Cohort Studies

  9. Cohort Studies (III) (b) Advantages: • No / little temporal ambiguity (suggests cause-effect relationship) • Calculation of incidence rates • Suitable for rare exposures • Factors associated with selection cannot influence disease status and hence the results. • Several outcomes can be studied, after follow-up starts. Cohort Studies

  10. Cohort Studies (IV) ( c ) Disadvantages (of prospective): • Expensive • Time-consuming • May be impractical • Loss to follow-up may affect sample-size Cohort Studies

  11. IDEAL COHORT An ideal cohort should be: • STABLE • COOPERATIVE • COMMITTED • WELL-INFORMED Cohort Studies

  12. Examples of Study Cohorts • General population • Selected occupational groups, e.g. health professionals (physicians, nurses, lab technologists, etc), manufacturers of mercury batteries for vehicles; asbestos workers, miners, etc Cohort Studies

  13. Sources of Cohorts • Population groups • Occupational settings (employment, medical records) • Hospital registers (medical records) • Death certificates Cohort Studies

  14. Follow-up Techniques • Periodical medical examinations and mailed questionnaires • Direct personal interviews or examinations • Videoconference, neighbors, friends and relatives • Lost persons can be traced through the letters, from their • relatives and friends • 5. Migrated cohort subjects can also be traced through travel and immigration authorities • 6. Dead persons - local or regional mortality registers • or death certificates Cohort Studies

  15. Problems during Follow-up • FOLLOW-UP OF A LARGE GROUP • LIMITED RESOURCES • TIME SCARCITY • PAUCITY OF TRAINED PERSONNEL • ATTRITION, LOSS ON FOLLOW UP • ETHICAL CONCERNS Cohort Studies

  16. Attrition Reduction • OBTAINING THE INFORMED CONSENT • RECORDING COMMITMENT TO CONTINUE AND COOPERATE IN THE STUDY • TRACING LOST SUBJECTS, TRYING TO INCLUDE THEM IN THE STUDY • CONSIDERING INFORMATION OF LOST PERSONS AT THE TIME OF ANALYSIS • KEEPING NON-RESPONSE AT A LOW LEVEL TO IMPROVE THE VALIDITY Cohort Studies

  17. Examples of Cohort Studies • POPULATION-BASED • CARDIOVASCULAR • CHILD HEALTH • SPECIAL EXPOSURES • NON-POPULATION BASED • OCCUPATIONAL – for convenience • OCCUPATIONAL – to study the occupation • HEALTH CARE SETTINGS • VETERANS Cohort Studies

  18. CARDIOVASCULAR DISEASE • USA: Framingham, MA; Tecumseh, MI; Evans county, GA; Muscatine, IA; Bogalusa, LA (children) • WHO MONICA (multi-center) • North Karelia, Norway Cohort Studies

  19. Framingham Study (1951 – present time) • 1ST Step: Selection of cohorts • Initially, 5209 subjects were enrolled into the study • Currently, thousands of people are followed up, both for cardiovascular risk factors (e.g. high serum cholesterol, smoking, hypertension, BMI, etc) and possible outcomes Cohort Studies

  20. SPECIAL EXPOSURES • Atomic Bomb Casualty Commission (ABCC): Hiroshima and Nagasaki survivors (effects of radiation) • Dutch famine survivors (effects of starvation) • Seveso (effects of dioxin exposure) Cohort Studies

  21. OCCUPATION BASED, TO STUDY EXPOSURES • Benzene-workers (leukemia) • Coke-oven workers (lung cancer) • Asbestos workers (lung cancer) • Radium dial painters (oral cancer) Cohort Studies

  22. Epidemiological Measures • Frequency measures: incidence; prevalence • Effect measures: risk ratio (relative risk); odds ratio • Impact measures: attributable fraction; prevented fraction Cohort Studies

  23. 2x2 Table Cohort Studies

  24. INCIDENCE RATES • INCIDENCE AMONG THE EXPOSED (NEW CASES AMONG THE SMOKERS) = (A/ A+B) • INCIDENCE AMONG THE NON-EXPOSED (NEW CASES AMONG THE NON-SMOKERS) = (C/ C+D ) Cohort Studies

  25. Risk Calculations • Relative Risk or Risk Ratio (RR) (RR) = [A / (A+B)] / [C / C +D)] (incidence in the exposed (smokers) / incidence in the non-exposed (non-smokers) • Attributable risk (AR = excess risk/ incidence among the exposed (AETIOLOGICAL FRACTION) • Population attributable risk (PAR) = incidence in the total population minus incidence among the non-exposed. Cohort Studies

  26. RR Interpretations • Unity: exposure has no effect on outcome in the studied population (cohort) • More than 1: exposed have a higher risk of developing the outcome, compared to the unexposed • Less than 1: either no relationship, or a “protective” one exists (e.g. effect of interventions, immunization, health education, management, etc) Cohort Studies

  27. Frequency & Effect Measures Cohort Studies

  28. References • Last JM. A dictionary of epidemiology. 5th edition. New York, Oxford, Toronto: Oxford University Press, 2008. • Gordis L. Epidemiology. Fourth edition. Philadelphia, London, New York, Sydney: W.B. Saunders, 2009. • Beaglehole R, Bonita R, Kjellstrom T. 2nd edition. Basic epidemiology. Geneva: WHO, 2006. • Friis RH, Sellers TA. Epidemiology for public health practice. Gaithersburg, MD: Aspen Publishers, 2009. • Kelsey JL, Thompson WD, Evans AL. Methods in observational epidemiology. New York: Oxford University Press, 1986. Cohort Studies

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