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BC Jung A Brief Introduction to Epidemiology - X (Epidemiologic Research Designs: Cohort Studies) Betty C. Jung, RN, MPH, CHES
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BC Jung Learning/Performance Objectives u To develop an understanding of: –What cohort studies are –The value of such studies –The basic methodology – Pros and Cons of such studies
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BC Jung Introduction Epidemiology studies the distribution of disease in a number of ways. The two major categories of epidemiological studies are: Observational and experimental studies. Most epidemiological studies are observational.
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BC Jung Epidemiological Study Designs u Observational Studies - examine associations between risk factors and outcomes (Analytical - determinants and risk of disease, and descriptive - patterns and frequency of disease) u Intervention Studies - explore the association between interventions and outcomes. (Experimental studies or clinical trials)
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BC Jung Research Designs in Analytic Epidemiology u Ecologic Designs Cross- Sectional Study u Case-Control Study u Cohort Study
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BC Jung Cohort Studies u “Motion Picture Studies” (Paffenbarger, 1988) u Forward looking. The most powerful of observational studies u Follow groups of individuals free from disease through a period of time u Quantified with relative risk/incidence rates/attributable risk
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BC Jung Examples of Cohort Studies u Framingham Heart Study u Body fat distribution and 5-year risk of death in older women (1993) - 15-unit increase in waist/hip circumference was associated with 60% greater relative risk of death. Waist/hip circumference ratio as a better marker than body mass index of risk of death in older women. u Vasectomy and Prostate Cancer - those who had a vasectomy and those who did not. Increase relative risk of those with vasectomy - increase risk of prostate cancer.
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BC Jung Historical Cohort Studies u Cohort formed in the past with period of follow-up ending also in the past u Used in occupational settings were population registers (payroll records) are available u Example: Atomic bomb blast survivors
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BC Jung Value u Gold standard for studying the association between a risk factor and outcome u Useful for studying incidence, risk factors, natural history or prognosis u Useful for studying multiple outcomes u Useful for looking at multiple exposures and their interactions
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BC Jung Cohort Study Design Direction of Inquiry Exposed Not Exposed Disease No Disease Time Population Disease People Without Disease
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BC Jung Cohort Study Design Define Population Non-randomizing ExposedNon-Exposed Disease No Disease DiseaseNo Disease 2015 2005 Concurrent 1995 Retrospect 1975 1985 1995
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BC Jung Methodology u Start with persons having the presumed cause (antecedent or exposure). BUT free from the effect (disease), and then wait for them to develop the effect u Comparison group - also free from disease, but who, also DO NOT have the presumed cause
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BC Jung Methodology u Cohort - group or aggregate of persons who have presume antecedent characteristics in common and observe the development or non-development of a given health outcome u Compare to those free of the disease or health outcome under study. Issue being at risk of repeated episode (i.e., Stroke, antecedents may different between prestroke 1 and prestroke 2
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BC Jung Cohort Study Measures u Cumulative Incidence - # new cases/at risk population u Incidence Density - # new cases/at risk person-time u Measures of association –Relative Risk –Odds Ratio
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BC Jung Strength of Association Relative Risk;(Prevalence); Odds Ratio Strength of Association 0.83-1.001.0-1.2 None 0.67-0.831.2-1.5 Weak 0.33-0.671.5-3.0 Moderate 0.10-0.333.0-10.00 Strong 10.0 Approaching Infinity Source: Handler,A, Rosenberg,D., Monahan, C., Kennelly, J. (1998) Analytic Methods in Maternal and Child Health. p. 69.
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BC Jung Pros u Can study situations where randomization is not possible u Time sequence strengthens the inference about cause (temporal relationship between exposure and outcome) u Only way to establish population-based incidence
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BC Jung Pros u Direct measure of incidence (risk) and prognosis (natural history) u Incidence rate is not influenced by the presence of the effect (outcome/disease) at the beginning of the study u Magnitude of a risk factor’s effect can be quantified u Can estimate the relative contribution of different (multiple) causes to the occurrence of the effect (disease or outcome)
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BC Jung Pros u Can count the number of prevalent cases, and new cases, as well as the number and proportion of cases that can be prevented u Information bias is decreased (i.e., selective recall/memory) u Can better measure the impact of confounding
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BC Jung Pros (Historical Cohort Studies) u Easier to create the cohort u Baseline measurements available u Follow-up has already occurred u Less costly and time consuming
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BC Jung Cons u Expensive u Not good for low-incidence (rare) diseases u Not good for chronic diseases with long latency u Time needed to conduct these studies u Unexpected changes to the environment can influence the association of disease and possible cause over time
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BC Jung Cons u Non-response/Migration bias – “loss to follow-up” u Selection bias – zero time not defined (lead-time bias) u Sampling bias u Ascertainment/Assessment bias of outcome (can be reduced by blinding/masking)
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BC Jung Cons u Information bias – data are different (i.e., different hospitals) – must to be comparable for exposed and unexposed u Confounding bias u Measurement bias - misclassification u Analytic/Observer bias – how data are analyzed and interpreted
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BC Jung Cons (Historical Cohort Studies) u Incomplete data sets u No control over the quality of the measurements that are available u Incomplete control of confounding
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BC Jung References u For Internet Resources on the topics covered in this lecture, check out my Web site: u http://www.bettycjung.net/ http://www.bettycjung.net/
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