BUSPH EP711 Final (Epidemiology)
Terms
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- Fixed population
- Membership is based on an event and is permanent
- Dynamic population
- Membership is based on a condition and is transitory
- 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 - Incidence
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-occurence of new cases over a specified period of time
-measured in a candidate population - 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 - 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 - Prevalence
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# of existing cases / # in total population
-point or period
-resource planning
- P=IR*D
(D = duration) - Organization in a 2 x 2 table
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Disease on x-axis
Exposure on y-axis - Absolute measures of comparison
- Risk difference, incident rate difference, etc.
- Risk difference
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RD=Re-Ru
e = exposed
u = unexposed - Population risk difference
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PRD=Rt-Ru
PRD=RDxPe
Pe = proportion exposed
-excess # of cases in total pop - 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 - Prevented factor
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PF=[(Ru-Re)/Ru]x100
-used when exposure is thought to be protective - Relative Risk
- RR=Re/Ru
- Excess Relative Risk
- ERR=(RR-1)x100
- Direct method of standardization requires...
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1) Age-specific rates in each group
2) Age structure of a standard population - Experimental study
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-studies prevention and treatment
-active manipulation - Observational study
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-studies causes, preventions, treatments
-passive
-cohort, case-control, - 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 - Case-control study
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-good for rare diseases
-examines multiple exposures
-exposure histories compared
-observational - Cross-sectional study
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-examines relationship between exposure and disease prevalence in a population at a single point in time
-observational - Ecological study
- -examines relationship between exposure and disease with population-level data
- Validity
- lack of bias and confounding
- Bias
- error committed by investigator in design or conduct that leads to a false association between exposure and disease
- Confounding
- reflects on the fact that research is on free living humans with unevenly distributed characteristics
- Precision
- lack of random error, which leads to false association
- Randomization
- an act of assigning or ordering that is the result of a random process
- Masking
- unawareness of treatment assignment of patient
- purpose of placebos
- to match as closely as possible the experience of the comparison group with that of the treatment group
- purpose of run-in period
- to ascertain which potential participants are able to comply with the study regimen
- Intent-to-treat
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-treatment assignment analysis
-all individuals in treatment group are analyzed - Efficacy analysis
- determines treatment effects under ideal conditions (i.e. compliance)
- Cohort
- a group of people with a common characteristic or experience
- Referent group
- unexposed, or comparison group
- Closed cohort
- fixed cohort with no losses to followup
- Possible timing of events in a cohort study
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-prospective
-retrospective
-ambidirectional - sources for comparison group
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-internal
-general pop
-comparison cohort - 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 - cohort study occurrence outcome measure
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-cumulative incidence
-incidence rate - purpose of case-control study
- investigate risk of disease in relationship to a wide variety of exposures
- problem with using prevalence in case-control studies
- impossible to tell if exposure is related to the inception of the disease, its duration, or a combination
- "would" criterion
- a member of the control group who gets the disease under study would end up as a case
- 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 - cons of using dead controls
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-not representative of living population
-more likely to have used tobacco, alcohol, drugs - How may biased results come about as an outcome from "special" controls (e.g. siblings)?
- if study hypothesis involves a shared activity
- survivor sampling
- select controls from non-cases at the end of the case diagnosis and accrual period
- risk set sampling
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-controls selected from population at risk as cases are being diagnosed
-may include future cases - 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 - odds ratio
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- prob will occur/prob will not occur
- (a/b)/(c/d) = (ad)/(bc) - Case-crossover study
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-used when risk of outcome has a short window
-cases serve as own controls - why use an experimental study?
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-high degree of validity from randomization (control for confounders)
-prevention or treatment with small effect - issues in experimental study design
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-noncompliance
-must maintain followup rates
-high costs
-reluctance to participate
-ethical issues (equipoise) - why use an observational study?
- study effects of a wider range of exposure
- why use a cohort study?
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-little is known about health consequences of exposure
-rare exposure - 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 - cons of case-control study
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-greater chance of bias
-not good with weak associations
-difficult to establish temporal relationship - cons of retrospective cohort study
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-minimal info is available on exposure, outcome, confounders
-difficult to establish temporal relationship - pros of cross-sectional study
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-highly generalizable
-short time
-low cost - 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 - why do a cross-sectional study?
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-for PH planning
-etiologic research - cons of ecological study
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-ecological fallacy or ecological bias: not generalizable to individuals
-inability to detect subtle or complicated relationships - pros of ecological study
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-quick
-inexpensive
-simple, easy to understand
-wider-range of exposure - a study is considered valid only when what 3 things are eliminated?
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-bias
-confounding
-random error - When evaluating a study for the presence of bias, investigators must:
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-identify its source
-estimate its magnitude
-assess its direction - selection bias
- an error due to systematic differences in characteristics between those who are selected for study and those who are not
- best way to avoid self-selection bias
- obtain high participation rates among both groups
- observation bias
- a flaw in measuring exposure or outcome data that results in different accuracy between comparison groups
- 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 - recall bias
- -differential level of accuracy in the info provided by compared groups
- 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 - avoiding interview bias
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-mask interviewers to subjects' disease or exposure status
-closed-ended questions - 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 - problem of loss to follow-up
- -reduces power of study to detect associations
- 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