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Cases Controls _____________________________________________________________________________________________ www.sph.unc.edu/courses/EPID168, © Victor J. Schoenbach 1999 8. Analytic study designs - 234 rev. 9/6/1999, 10/7/1999, 12/17/1999
Validity The validity of a case-control study requires that: Cases in the study adequately represent the relevant cases (the population of cases about whom inferences are to be made) with respect to the variables of interest (notably, prevalence of exposure). This depends upon whether the cases available do in fact reflect the rates of disease in exposed and unexposed individuals undistorted by differential manifestation, detection, or short-term survival (e.g., selective survival, access to care, detection bias); Controls accurately reflect the exposure proportions in the study base (the source population for the cases). For example, hospitalized controls may overrepresent exposures associated with hospitalization for other conditions. Both of these requirements, especially the latter, can be difficult to ensure. Therefore, case-control studies are regarded as highly susceptible to bias from problems with the: Identification of cases Reliance on medical care system Often miss subclinical cases (detection bias?) Can miss rapidly fatal cases (selectively?) Selection of controls Selection of controls can determine the study results Which controls are appropriate is often not obvious Trade-off between sampling and data collection Hospitalized controls, community controls, dead controls Controls may be reluctant to cooperate Measurement of exposure for cases and controls Reliance on recall or records (differential?) Effect of disease on exposure assessment Effect of disease on exposure (confounding by indication) There is also the thorny problem of establishing temporality, i.e., did the exposure precede the disease? Interpretability of the odds ratio Why does the OR from the cases and controls we have assembled estimate anything in the population? Consider what the cells in the table below represent. Assume that the cases were selected as newly occurring cases of endometrial cancer over a period of time in a defined _____________________________________________________________________________________________ www.sph.unc.edu/courses/EPID168, © Victor J. Schoenbach 1999 8. Analytic study designs - 235 rev. 9/6/1999, 10/7/1999, 12/17/1999
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Understanding the Fundamentals of E
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Preface Introductory epidemiology c
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Table of Contents Chapter (in Acrob
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1. Introduction Definition, charact
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problems (Schwartz and Carpenter, 1
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HIV - a new or newly-recognized vir
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d = principal investigator's degree
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Analytic studies typically involve
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Observational sciences especially a
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McGavran EG. What is public health?
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2. An evolving historical perspecti
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Time line for the history of public
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as a one-room bacteriology laborato
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− Health surveys − Research fun
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his scientific peers of the correct
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Kuhn, Thomas S. The structure of sc
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3. Studying populations - basic dem
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The demographic balancing equation
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ates. Similarly, bulges in the repr
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When the baby boom cohort retires 1
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The pervasiveness, strength, viciou
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Total fertility rate (TFR) Standard
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Life expectancy The technique, of u
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Excerpt from the U.S. 1993 abridged
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At the other end of the life table,
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to change, hopefully to decline. If
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Thought question: Professors typica
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Descriptive studies and surveillanc
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4. The Phenomenon of Disease Concep
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Classification is the foundation As
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As we gain more sophisticated under
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To recapitulate the above discussio
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Classifying cause of death Since mo
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Laboratory and other objectively me
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distribution but are not diabetic (
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program in the general population,
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Infectious disease Incubation "time
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the direct effects of carcinogenic
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patient outcomes. The design and ev
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detected by screening will appear b
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Morrison, Alan S. Screening in chro
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e relevant. Compromises might be fo
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educational attainment by the numbe
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Distributions - the fuller picture
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Per capita income: Should health ca
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Deaths of children < 1 year of age
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No. of persons with senile dementia
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Incidence Immigration Relationship
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Examples: arthritis, cholelithiasis
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Cohort - entrance into the populati
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Sample calculation: 200 people free
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• The units of time must be state
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Calculation of person-time in a coh
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Original Calculation of person-time
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average risk for a member of the co
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However: Nurses: 601 cases/89,538 w
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treated. If the rate at which patie
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Incidence and prevalence in a popul
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small prevalence. So if the seropre
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Appendix on weighted averages Becau
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Appendix on exponents and logarithm
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and epidemiology. The notation ln(x
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3. For the following hypothetical d
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c. ________________ 6 fatalities /
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Measuring disease and exposure - As
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5. a. Flow Diagram Population of si
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New cases Q ID = ------------------
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Overview 6. Standardization of rate
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differences between the groups in f
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∑ (stratum-specific rates × stan
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especially important when comparing
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copy this cell to the other columns
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where the RR k are the stratum-spec
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and the observed number of deaths i
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Directly standardized rate for A =
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5,022 Indirectly standardized = —
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the situation among older age group
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Appendix on Standardized Mortality
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across age strata, then the damage
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d. How would you feel about the con
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Cases per 100,000 Mean annual incid
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d. dt Indirectly standardized rates
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The second ingredient for an standa
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Group US rate /100,000 County pop.
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Crude rates are comparable because
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The “Big Picture” 7. Relating r
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CHD and oral contraceptives in Brea
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Some basic measures Before diving i
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esophageal cancer? First, if we did
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ought in here as well. When we cont
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Exposure Disease Yes No Total Yes a
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Relation of the odds ratio to the r
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the case-control study estimates th
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If the data had come from a cross-s
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Excess risk = CIR - 1 = 1.0 (i.e.,
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strength of the relationship (e.g.,
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drawn on ordinary graph paper. Howe
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Measures of Impact Concept Relative
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Attributable risk proportion The AR
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Page 201 and 202: Incidence Diagrammatic representati
Page 203 and 204: 1. high frequency of disease 2. pow
Page 205 and 206: With this diagram and notation we c
Page 207 and 208: vaguer) “attributed to”. Incide
Page 209 and 210: Summary There are three categories
Page 211 and 212: CIR can be directly estimated if th
Page 213 and 214: I0 I = ——————— 1 + P1
Page 215 and 216: PD|e PE (RR - 1) PD|e PE (RR - 1) P
Page 217 and 218: Walter, Stephen D. Choice of effect
Page 219 and 220: a. Quit rates were measured as the
Page 221 and 222: 1. Definitions: Relating risk facto
Page 223 and 224: There is an extremely strong associ
Page 225 and 226: 8. Analytic study designs The archi
Page 227 and 228: Progression of types of studies In
Page 229 and 230: Cross-sectional A cross-sectional s
Page 231 and 232: Follow-up studies Along with case-c
Page 233 and 234: of interest community-level influen
Page 235 and 236: ecological approach may be the appr
Page 237 and 238: What measures can be estimated from
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Page 241 and 242: individuals, this relationship does
Page 243 and 244: Odds Ratio (OR) Exposure odds in ca
Page 245 and 246: Simple randomization - assignment o
Page 247 and 248: The relationship of an attribute to
Page 249: (a / m1) / (b / m1) ad ORe (Exposur
Page 253 and 254: Since the strategic advantage of th
Page 255 and 256: c = f0N1 = number of exposed noncas
Page 257 and 258: experienced the event under study.
Page 259 and 260: approach, the odds ratio computatio
Page 261 and 262: Exposure Yes No Total Cases A B M1
Page 263 and 264: Miettinen, Olli S. The clinical tri
Page 265 and 266: Mantel, N. and Haenszel, W.: Statis
Page 267 and 268: Peto, R., Pike, M.C., Armitage, P.,
Page 269 and 270: 4. The authors state that "There we
Page 271 and 272: 5. In contrast to many earlier inve
Page 273 and 274: In this study the use of hospitaliz
Page 275 and 276: Follow-up must be longer for any gi
Page 277 and 278: While acknowledging the very import
Page 279 and 280: world. Even as late as the 1950's,
Page 281 and 282: Causal Inference Direct observation
Page 283 and 284: data on the effects of radiation on
Page 285 and 286: But excluding an explanation based
Page 287 and 288: Strength of the association Pronou
Page 289 and 290: Experimental evidence Certain types
Page 291 and 292: To issue an indictment, the grand j
Page 293 and 294: 10. Sources of error A systematic f
Page 295 and 296: the data in the table. (Note that t
Page 297 and 298: associations. So the same terms may
Page 299 and 300: "Negative bias" - The observed meas
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Classifying sources of bias Inspite
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The study population is a subset of
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This estimate of the prevalence of
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unmatched control groups will presu
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The odds ratio for this table is 3.
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Selection bias in cohort studies Se
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Information bias Information bias r
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following chapter). The concept of
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P h y s i c i a n A Comparison of d
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item). Alpha values of 0.80 are are
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Hypothetical scenario showing effec
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presumably classified as informatio
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Appendix 1 Formulas to see the effe
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Algebraically, _ Pr(E|D') = — the
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Gregorio, David L.; James R. Marsha
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Sources of error - Assignment 1. Th
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Alpha (α) is the probability by wh
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Sources of error - Assignment solut
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3. ii) Selective survival--if OC us
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a. Flow diagram: 1960-1962 1967-196
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Introduction 11. Multicausality: Co
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estimate of what the outcomes would
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A hypothetical example (with apolog
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Questions to ask: There are many as
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Confounding arises from unequal dis
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CHD Incidence by Behavior Pattern a
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In this way, the 1,129 Type A's wit
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exposure. If we assume a baseline r
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Further insight can be gained by co
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Table 7a Colon cancer and drinking
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atio of the observed number of expo
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ural-specific exposure prevalences,
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eing infected, and the more of them
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studies deal with a delimited geogr
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University of Massachusetts at Amhe
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similarly distributed in cases and
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"There have been many important ste
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MAIN POINTS Confounding is a disto
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Greenland, S. and Neutra R.: Contro
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Appendix The following discussion i
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i n1i wi = ----- If exposure is unr
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Multicausality: Confounding - Assig
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4. The following table, published i
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1. Multicausality: Confounding - As
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link between HCS and MI to ensure t
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Multicausality 12. Multicausality:
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Interdependent effects The precedin
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epidemiologic data. Put simply, the
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BP 0 These two lines are parallel;
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ean intake and B representing genet
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Fourth, proportion of disease attri
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Incidence and incidence ratios of f
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% with evidence of impaired red blo
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cancer rate among the light smokers
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Age standardized lung cancer rates
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R11 - R00 = R10 - R00 + R01 - R00 (
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and R111 = R100 × R010 × R001 / (
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R110 = R100 × R010 / R000 (multipl
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Risk ln(risk) 0.19 -1.67 0.13 -2.01
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have clinical or public health sign
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Baseline Characteristics Associated
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Weiss, Noel S. Accounting for the m
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A. Synergism apparently exists in t
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5. Walker (International Journal of
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(or equivalently, the rate for pers
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impact. Synergism in this sense imp
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13. Multicausality - analysis appro
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very far on instruments but needs t
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Summarizing the relationships Often
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We saw in the chapter on effect mod
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So the multiplicative model for joi
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misleading summary. These circumsta
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w1 CID1 + w2 CID2 Summary CID = —
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c d bcn0 + adn1 Var(ln(CIR)) ≈
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Kleinbaum, Kupper, and Morgenstern;
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1 + 1 + 1 3 ORMH = —————
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Risk = R00 + Obesity effect Obesity
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two (or more) factors that "interac
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OR = exp(β3) β3 is the difference
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Other regression models [Optional f
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Know the epidemiologic meaning of t
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14. Data analysis and interpretatio
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and questionable data. Monitoring p
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presence of particular proteins (e.
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4. Count - the number of entities,
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Algorithms - A procedure that uses
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Imputation causes the least distort
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Prenatal exposure to diethylstilbes
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Possible outcomes (Colors of pairs
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In a strict decision-making mode, t
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C = area between -z and +z D = area
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two-tailed 5% significance level).
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If the p-value is not small (i.e.,
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OR we observed was greater than 1.0
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Large trials (e.g., 250 deaths) Tru
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The concept behind the confidence i
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Formulas for confidence intervals f
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Woolf SH, Battista RN, Anderson GM,
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Stallones, Reuel A. The use and abu
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C - Clustered observations, which d
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For example, suppose that n, the sa
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* See previous table Margin of erro
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Even in randomized trials problems
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Pr(H|T) is therefore a function of
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That information gives you pause. C
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Part II 1. In table 2 from the Rose
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If the OR for Current OC use were t
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6. There are no interaction (produc
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24. Submit manuscript from previous
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Cooperative agreement A cooperative
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Assurances by the applicant and her
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The new element is the grant award.
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1. What needs to be done? 2. How lo
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Postaward Final report What to do
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2. maintain close supervision of th
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Publication Epidemiology Working Gr
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Weed, Douglas L. Between science an
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1.3 Specific Objectives of Data Man
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engender a basic conflict between t
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1.4.2 Integration Integrate the dat
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2. Consistency of the protocol's im
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2. Coding of a sample of data forms
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2.3 The steps in the Editing proces
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Although it is rare to use more tha
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The first stages of data analysis s
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have less power to exclude the stat
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greater nonresponse by heavy drinke
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Imputation is then carried out as f
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(normal) distribution in the popula
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Appendix **************************
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___________________________________
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Public health approach The public h
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health, particularly at the world l
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Causative or preventive agents Scur
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Human behavior is also biology The
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crowded, under-resourced urbanized
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Bibliography Annas, George J.; Leon
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18. Overview and Conclusion A look
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Epidemiology is expanding In recen
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More reliance on "soft money" - res
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__________ This is not a new challe
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