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77 Cards in this Set
- Front
- Back
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Fixed population
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Membership is based on an event and is permanent
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Dynamic population
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Membership is based on a condition and is transitory
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3 factors to consider in measurement of disease frequency
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1. # of people affected
2. # of people in population from which cases arise 3. length of time followed |
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Incidence
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-occurence of new cases over a specified period of time
-measured in a candidate population |
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Cumulative Incidence
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# new cases/ # in candidate population over a specified period of time
-used in fixed populations, few losses to followup -research causes, prevention, treatment of disease - CI=IR*t |
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Incidence Rate
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# new cases / person-time in candidate population
-measure of transition between health/disease of dynamic populations -research on causes, prevention, treatments |
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Prevalence
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# of existing cases / # in total population
-point or period -resource planning - P=IR*D (D = duration) |
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Organization in a 2 x 2 table
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Disease on x-axis
Exposure on y-axis |
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Absolute measures of comparison
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Risk difference, incident rate difference, etc.
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Risk difference
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RD=Re-Ru
e = exposed u = unexposed |
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Population risk difference
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PRD=Rt-Ru
PRD=RDxPe Pe = proportion exposed -excess # of cases in total pop |
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Attributable proportion among the exposed
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APe=[(Re-Ru)/Re]x100
-etiologic fraction -proportion of disease among exposed that would be eliminated if exposure were eliminated |
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Prevented factor
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PF=[(Ru-Re)/Ru]x100
-used when exposure is thought to be protective |
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Relative Risk
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RR=Re/Ru
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Excess Relative Risk
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ERR=(RR-1)x100
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Direct method of standardization requires...
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1) Age-specific rates in each group
2) Age structure of a standard population |
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Experimental study
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-studies prevention and treatment
-active manipulation |
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Observational study
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-studies causes, preventions, treatments
-passive -cohort, case-control, |
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Cohort study
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-examines multiple health effects
-good for rare exposures -followed according to exposure levels and followed for disease occurrence -observational |
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Case-control study
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-good for rare diseases
-examines multiple exposures -exposure histories compared -observational |
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Cross-sectional study
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-examines relationship between exposure and disease prevalence in a population at a single point in time
-observational |
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Ecological study
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-examines relationship between exposure and disease with population-level data
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Validity
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lack of bias and confounding
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Bias
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error committed by investigator in design or conduct that leads to a false association between exposure and disease
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Confounding
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reflects on the fact that research is on free living humans with unevenly distributed characteristics
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Precision
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lack of random error, which leads to false association
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Randomization
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an act of assigning or ordering that is the result of a random process
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Masking
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unawareness of treatment assignment of patient
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purpose of placebos
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to match as closely as possible the experience of the comparison group with that of the treatment group
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purpose of run-in period
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to ascertain which potential participants are able to comply with the study regimen
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Intent-to-treat
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-treatment assignment analysis
-all individuals in treatment group are analyzed |
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Efficacy analysis
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determines treatment effects under ideal conditions (i.e. compliance)
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Cohort
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a group of people with a common characteristic or experience
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Referent group
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unexposed, or comparison group
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Closed cohort
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fixed cohort with no losses to followup
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Possible timing of events in a cohort study
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-prospective
-retrospective -ambidirectional |
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sources for comparison group
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-internal
-general pop -comparison cohort |
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pros and cons of using Health Records as sources of info
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pros: low expense, high accuracy
cons: other key characteristics missing |
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cohort study occurrence outcome measure
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-cumulative incidence
-incidence rate |
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purpose of case-control study
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provide information on exposure distribution in population
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problem with using prevalence in case-control studies
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impossible to tell if exposure is related to the inception of the disease, its duration, or a combination
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"would" criterion
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a member of the control group who gets the disease under study would end up as a case
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advantage to using hospital controls
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-easy to identify
-good participation rates -less expensive to identify -come from the same population -recall is similar to cases |
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cons of using dead controls
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-not representative of living population
-more likely to have used tobacco, alcohol, drugs |
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How may biased results come about as an outcome from "special" controls (e.g. siblings)?
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if study hypothesis involves a shared activity
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survivor sampling
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select controls from non-cases at the end of the case diagnosis and accrual period
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risk set sampling
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-controls selected from population at risk as cases are being diagnosed
-may include future cases |
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case-based or case-cohort sampling
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-select controls from population at risk at the beginning of case diagnosis and accrual period
-may include future cases |
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purpose of case-control study
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investigate risk of disease in relationship to a wide variety of exposures
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odds ratio
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- prob will occur/prob will not occur
- (a/b)/(c/d) = (ad)/(bc) |
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Case-crossover study
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-used when risk of outcome has a short window
-cases serve as own controls |
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why use an experimental study?
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-high degree of validity from randomization (control for confounders)
-prevention or treatment with small effect |
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issues in experimental study design
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-noncompliance
-must maintain followup rates -high costs -reluctance to participate -ethical issues (equipoise) |
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why use an observational study?
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study effects of a wider range of exposure
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why use a cohort study?
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-little is known about health consequences of exposure
-rare exposure |
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why use a case-control study?
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-little is known about disease etiology
-less time, $ than a cohort -rare disease -diseases with long induction and latent periods -when exposure data is difficult to obtain |
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cons of case-control study
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-greater chance of bias
-not good with weak associations -difficult to establish temporal relationship |
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cons of retrospective cohort study
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-minimal info is available on exposure, outcome, confounders
-difficult to establish temporal relationship |
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cross-sectional study
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-measures exposure prevalence in relation to disease prevalence
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pros of cross-sectional study
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-highly generalizable
-short time -low cost |
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cons of cross-sectional study
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-no temporal inference when exposure is changeable
-bias: identifies high proportion of cases w/ long duration -healthy-worker effect |
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why do a cross-sectional study?
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-for PH planning
-etiologic research |
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ecological study
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-examines rates of disease in relation to a factor described on a population level
-identifies groups by place/time |
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cons of ecological study
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-ecological fallacy or ecological bias: not generalizable to individuals
-inability to detect subtle or complicated relationships |
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pros of ecological study
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-quick
-inexpensive -simple, easy to understand -wider-range of exposure |
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a study is considered valid only when what 3 things are eliminated?
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-bias
-confounding -random error |
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When evaluating a study for the presence of bias, investigators must:
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-identify its source
-estimate its magnitude -assess its direction |
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selection bias
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an error due to systematic differences in characteristics between those who are selected for study and those who are not
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best way to avoid self-selection bias
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obtain high participation rates among both groups
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observation bias
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a flaw in measuring exposure or outcome data that results in different accuracy between comparison groups
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Key features of observation bias
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-occurs after subjects have entered study
-pertains to data collection -it often results in incorrect classification |
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recall bias
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-differential level of accuracy in the info provided by compared groups
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ways to minimize recall bias
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-select a diseased control group
-comprehensive questions for exposure ascertainment -self-administered questionnaire -rely on measurements and records |
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avoiding interview bias
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-mask interviewers to subjects' disease or exposure status
-closed-ended questions |
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tips for writing understandable questions
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-avoid ambiguous wording
-avoid 2 questions in one -avoid jargon -avoid impossible questions -avoid questions that are not self-explanatory -avoid questions w/ too many ideas |
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problem of loss to follow-up
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-reduces power of study to detect associations
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misclassification
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-"measurement error"
-occurs when broad definitions are used -can reduce by using multiple measurements, validation, or using sensitive and specific criteria |
