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Analytic Epidemiology Analytic Studies: Observational Study Analytic Studies: Observational Study Designs Cohort Studies Analytic Epidemiology Prospective Cohort Studies Retrospective (historical) Cohort Studies Part 2 Case-Control Studies Nested case-control Case-cohort studies Dr. H. Stockwell Case-crossover studies Determinants of disease: Analytic Epidemiology Epidemiology: Risk factors Identifying the causes of disease A behavior, environmental exposure, or inherent human characteristic that is associated with an important health Testing hypotheses using epidemiologic related condition* studies Risk factors are associated with an increased probability of disease but may Goal is to prevent disease (deterrents) not always cause the diseases *Last, J. Dictionary of Epidemiology Analytic Studies: Cohort Studies Panel Studies Healthy subjects are defined by their Combination of cross-sectional and exposure status and followed over time to cohort determine the incidence of disease, symptoms or death Same individuals surveyed at several poiiiints in time Subjects grouped by exposure level – exposed and unexposed (f(reference group, Can measure changes in individuals comparison group) Analytic Studies: Case-Control Studies Nested case-control studies A group of individuals with a disease Case-control study conducted within a (cases) are compared with a group of cohort study individuals without the disease (controls) Advantages of both study designs Also called a retrospective study because it starts with people with disease and looks backward for ppprevious exposures which might be relevant to the development of the disease Other case-control designs Intervention Studies Case-cohort studies – controls sampled from entire source population – those at Also called Experimental studies risk at the start of the study Randomized Controlled Clinical Trials Case-crossover – cases serve as own Community or field trials controls- good for events that have acute onset times Randomized Controlled Trials Community Trials Preventive Random allocation is at community level Intervention or other group such as a school Therapeutic Randllfldom allocation of volunteers to experimental or control procedure to dfldetermine impact of experimental exposure on outcome Why do we do an analytic study? Measures of Association Most often to look for a relationship Goal: between an exposure and disease A single measure that estimates the association between an exposure and the risk of developing disease Does exposure to factor X Calculate ratios of the measures of disease freqqyuency i/dikfdiincrease/decrease your risk of disease (usually incidence) This ratio is called a relative risk Y? or Calculate the difference between the two measures Need a measure of this association of disease frequency called risk difference or attributable risk Measures of association Relative Risk Estimates Relative risk and risk difference/ Cohort studies- calculate relative risk attributable risk are the two most Case-controldl studies – calllculate odds frequently used measures used in ratio to estimate relative risk epidemiology Cross-sectional studies – calculate the Together they provide a more complete prevalence rate ratio pihihicture than either measure al one Cohort Studies: Calculating the Relative Risk Two kinds of Incidence • It indicates the likelihood of developing the Cumulative Incidence disease in the exposed group relative to those not exposed. Number of new cases of disease over a specified time period Total population at risk • It is the ratio of the incidence of disease among exposed group divided by the incidence of disease among non-exposed Incidence Density Number of new cases of disease over a specified time period • The RR estimates the strength of an Total person-time of observation association between exposure and disease. • RR= IE / I0 Relative Risk Risk Ratio • If the incidence measure being used is Disease/Outcome cumulative incidence , the relative risk is called Risk ratio = a / a+b D D the risk ratio c / c+d E ab Exposed E cdd • If the incidence measure being used is CI incidence density, the relative risk is called the exposed = rate ratio CI unexposed Risk Ratio Meaning of RR 1.0 = no association: There is no relation between the Exposure and Disease Rate ratio = a / PYe c / PYu > 1.0 = Exposure is a risk factor for the Disease: Persons with the exposure are more likely to get Disease ID exposed = < 1.0 = E is inversely related to D: Persons with ID unexposed the exposure are less likely to get Disease The relative risk is RELATIVE Relative Risk to the baseline incidence The relative risk estimate is relative to CIE = .0026 Risk ratio (RR) = 1.0 the baseline incidence CI0 = .0026 CI = .49 E Risk ratio (RR) = 1.0 CI0 = .49 5 IDE = .062 per 100 PY 5 Rate ratio (RR) = 1.0 ID0 = .062 per 100 PY Cohort Study: Risk Ratio Risk Ratio Cumulative incidence= OC Use Infection OC Use: Infection Yes No Total CI in Exposed/CI in Unexposed Yes No Total Yes 27 455 482 Yes 27 455 482 No 77 1831 1908 27/482= .056 (CI in Exposed) Total 104 2286 2390 No 77 1831 1908 77/1908=.040 (CI in Unexposed Total 104 2286 2390 Risk Ratio= .056/.040 = 1.4 Rate Ratio Calculate the rate ratio Question: HRT use and CHD Incidence density among E CHD RR = Yes No Incidence density among E + 54308.7 Study sample: 30 HRT + HRT -- 60 d 51477.5 30 women taking HRT developed CHD after Rate ratio = ID = 30 / 54308.7 = .0005524 54,308.7py of follow-up E ID0= 60 / 51477. 5 = .0011655 RR = 0.0005 / 0.00116 = .47 or .5 60 women not taking HRT developed CHD after 51,477 .5py of follow -up Interpretation: women using HRT had only half (.5) the risk of developing CHD as did non users. Cross-sectional studies: Calculating the prevalence rate ratio Prevalence Rate Ratio PRe=27/61=.443 Prevalence rate among exposed Exposure Outcome smoking Depression No Total PRu=26/88=.295 Yes Prevalence rate among unexposed PRR= . 443/. 295 Yes 27 (a) 34 (b) 61 =1.5 The prevalence of the outcome (depression) is No 26 (c) 62 (d) 88 1.5x more common among those exposed Total 149 (smokers) than those unexposed (non- smokers) NO AUDIO ON THIS SLIDE Case-control Studies Review Quiz 2: Rationale Question 1 Cannot calculate incidence in case- control studies , so cannot calculate 450 /(450 + 20) Prevalence Rate Ratio = = 1.27 relative risk 1150 /(1150 + 380) Estimate relative risk in case-control Question 2 studies byyg calculating the odds ratio This cohort study measured cumulative incidence so (OR) you would calculate a risk ratio. 160 /(160 + 80) 0.667 RR = = = 6.009 40 /(40 + 320) 0.111.
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