Shared Flashcard Set

Details

Epi review
JHSPH09
130
Medical
12/19/2009

Additional Medical Flashcards

 


 

Cards

Term
3 general types of bias
Definition
1. Confounding, 2. Misclassification/Info bias, 3. Selection bias
Term
If bias is conservative, effect will be biased towards...
Definition
the null
Term
P-value definition
Definition
Probability that the test statistic is more extreme than the observed, given that the Ho is true
Term
Name 4 threats to causal inference
Definition
1. Lack of precision, 2. Lack of internal validity, 3. Incorrect asses. of direction of causality, 4. Lack external validity
Term
What is the counterfactual definition of confounding?
Definition
The freq. of the disease in the unexposed is different from the freq. of disease in the counterfactual exposed pop. had they been unexposed
Term
What are the 3 necessary criteria to define confounding?
Definition
1. Confounder is a cause of outcome, 2. Confounder is associated with exposure, 3. Confounder is no affected by exposure (not in causal pathway)
Term
What is a backdoor pathway?
Definition
A non-causal association between exposure and outcome. They begin with arrows pointing into the exposure
Term
What is Simpson's paradox?
Definition
Non collapsability does not assure non- confounding
Term
By how much is commonly accepted that an adjusted OR differ from a crude OR to say that confounding is present?
Definition
> 10%
Term
Name 3 methods to control confounding in design stage
Definition
!. Randomization, 2. Restriction, 3. Matching
Term
Name 3 methods to control confounding in the analysis
Definition
1. Standarization, 2.Stratification, 3. Multivariate models
Term
How do you standardize with the direct method?
Definition
Multiply observed incidence rates by age by a standard population weight
Term
Name limitations of standardized rates
Definition
1. Need to know stratum spec. rates, 2. Absolute magnitude depends on choice of standard pop., 3. Adjusted rates can only be compared across pops if same standard pop is used
Term
How do you calculate standardized mortality rates (SMR)?
Definition
SMR= observed/expected x 100
Term
What is non-differential error?
Definition
Measurement error doesn't depend on levels of other variables (error is the same for outcome, confounder, etc.)
Term
What is independent error?
Definition
Measurement error in a variable is not associated with errors in measuring other variables
Term
Non-differential, independent measurement errors bias estimate towards...
Definition
the null
Term
What is regression towards the null?
Definition
When a variable is measured with random error and we select participants with observed extreme values, their true underlying values are on average closer to the pop.mean
Term
When can regression towards the mean be important?
Definition
in pre/post studies. An association may appear for the intervention just because there is regression towards the mean
Term
Name 2 biases that can result in differential measurement error
Definition
1. Surveillance bias, 2. Diagnostic bias
Term
Define selection bias
Definition
When selection into study is affected both by exposure and cause of outcome
Term
What is emigrative selection bias?
Definition
When losses to follow-up are differential
Term
What types of biases can result from a healthy worker effect?
Definition
Selection bias and counfounding
Term
What is confounding by indication?
Definition
A variable is a risk factor for a disease among nonexposed persons and is associated with the exposure of interest in the population from which the cases derive
Term
Define RCT
Definition
Cohort study in which exposure assignment is determined by study
Term
What is blocking?
Definition
Technique used in RTC to avoid serious imbalances in # of participants in each group
Term
What is a phase 1 RTC?
Definition
Concerns safety
Term
What is a phase 2 RTC?
Definition
Efficacy study
Term
What is a phase 3 RCT?
Definition
Randomized controlled study
Term
What is a phase 4 RCT?
Definition
Post-marketing study
Term
What is a factorial trial for?
Definition
To test >1 intervention compared to a control in a single trial
Term
What is intention to treat analysis?
Definition
To analyze groups as they were assigned (analyze as randomized)
Term
What is a risk set?
Definition
all cohort members with the same opportunity for observing the event, and that have been followed for the same amount of time without the event
Term
How do you assemble a nested-case control study?
Definition
!. Identify cases, 2. Reorder according to outcome time scale, 3. For each case, identify risk set, 4. Randomly select controls from risk set
Term
What is incidence density sampling?
Definition
Compare cases to non-cases (at event time) among cohort members
Term
What is cumulative incidence sampling?
Definition
Select all cases. Those without outcome at the end of study period are controls
Term
How do you assemble a case-cohort?
Definition
1. Select a subcohort at start of risk period, 2. Identify cases in subcohort, 3. Include remaining cases outside subcohort
Term
What is an absorbent state?
Definition
Once exposed, always exposed (no need for more assessments)
Term
In a case-cohort, can cases outside the cohort be controls?
Definition
NEVER. Not in risk set
Term
In a nested case-control, can a case be a control?
Definition
YES. A case may serve as a control for a previous case
Term
In a nested case-control, can an individual be a control at several times?
Definition
YES
Term
In a nested case-control, outcome is/are...
Definition
specific
Term
In a case-cohort, the outcome is/are...
Definition
single or multiple
Term
Strengths of the case control studies
Definition
Efficient, shorter time period, good for rare diseases, may assess multiple outcomes
Term
In case control studies, when does the OR approximate the RR?
Definition
When incidence sampling is used
Term
How do you avoid selection bias in case control studies?
Definition
By selecting cases independent of exposure
Term
What is Berkson's bias?
Definition
Type of selection bias in hospital based C-C studies. Combination of exposure and disease increases risk of admission to hospital
Term
What type of sampling better represents person-time exposure in C-C studies?
Definition
Incidence density sampling. Controls are selected at the time of each incident case
Term
How do you assemble a case control study?
Definition
1. Identify target and source pop, 2. Recruit cases and controls, 3. Ascertain exposure status
Term
Is target and source population always the same for cases and controls?
Definition
Target pop. should be the same, source can differ
Term
What are elegibility criteria?
Definition
Characteristics that define the target and source pop. Exclusion+ inclusion criteria
Term
What is diagnostic bias?
Definition
Awareness of a possible association between exposure and outcome, where exposed individuals are followed more closely and detection of cases is thus increased
Term
What are concerns when using prevalent cases?
Definition
1. Survivor cases, 2. Change of exposure due to disease, 3. Reverse causation
Term
Name sources for control selection
Definition
1. Same as cases, 2. Geographic related (neighbors), 3. Random digit calling, 4. friends, family, 5. Same hospital/clinic
Term
What is individual matching?
Definition
Pairing or grouping controls to cases by a known risk factor
Term
How do you calculate a match OR?
Definition
OR=#pairs expo case and unex control/ #pairs case unexpo and control expo
Term
Can you produce a selection bias with matching?
Definition
YES. If selecting on a confounder in case control studies
Term
On what depends the bias of the crude OR compared to a matched OR?
Definition
On how much the matched factor is associated with the exposure
Term
What is frequency matching?
Definition
Selection of controls based on freq. of case characteristic so that they have the same distribution of potential confounder
Term
What are limitations of matching?
Definition
Effect of match var on outcome cannot be assessed, costly, can overmatch, can match on exposure or surrogate
Term
What is the invariance of the OR mean?
Definition
OR of exposure comparing diseased & non-diseased = OR of disease comparing expo & un expo
Term
Can the RR be calculated from the OR?
Definition
RR= OR/(1-po) + (po-OR)
Term
The OR of disease is the ______ of the OR of non-disease
Definition
reciprocal (1/OR)
Term
2 definitions of effect modification
Definition
1. The causal effect of exposure is different by the categories of other variable(s) 2. Joint causal effect of 2 vars is different from what would be expected by adding the independent effects
Term
Important characteristics of effect modification
Definition
1. Scale dependent (additive & multiplicative), 2. Is symmetrical, 3. Causal effect that should be estimated
Term
Type of scale (multiplicative or additive) more relevant for PH
Definition
Additive
Term
Type of scale (additive or multiplicative) that better describes observed patterns of disease
Definition
Multiplicative
Term
What type of error is increased when testing for interaction?
Definition
Both type I (increased in multiple comparisons) and type II more diff, to identify true interactions because of great random variability)
Term
What are Hill's criteria for causation?
Definition
1. Strength of association, 2. Temporality, 3. Plausibility, 4. Consistency, 5. Specificity, 6. Bio gradient, 7. Coherence, 8. Experiment, 9. Analogy
Term
What is a sufficient cause?
Definition
Minimal set of conditions and events that are sufficient for the outcome to occur. A complete causal mechanism
Term
What is a necessary cause?
Definition
A component cause that appears in every sufficient cause
Term
What is herd immunity?
Definition
Resistance of a group to an attack by a disease to which a large proportion of members are immune
Term
What is a DAG (Directed Acyclic Graph)?
Definition
Set of arrows drawn along a time line, characterizing causal and temporal relationships between variables
Term
What is the Study population?
Definition
Group under observation in study with the same exposure-disease assoc. (although not necessarily representative)
Term
What is the Target population?
Definition
Etiologic goal of Epi research, not always numerable
Term
What is the Source population?
Definition
Group expected to have the same expo-disease association and can be numerated
Term
If individuals enter together, non are added in follow up and exit for endpoint only, what type of cohort is it?
Definition
Closed on entry and exit
Term
What is an open cohort based on entry and exit?
Definition
A cohort where ind. enter at different times and exit for various reasons
Term
What are censored individuals?
Definition
!. Those that didn't experience endpoint of interest during follow up, 2. Those that cannot be observed because lost, dead, schedule end
Term
What is person-time?
Definition
Cumulative time spent by each individual at risk in the population
Term
What is a lagged exposure?
Definition
The period of exposure time not relevant for outcome
Term
Why is etiological relevant exposure measured?
Definition
Counting wasted exposure tends to dilute associations
Term
3 reasons for apportioning person-time
Definition
1. Risk varies with age, 2. Risk varies over time (secular trend), 3. Risk for disease may vary over expo levels
Term
What is the cumulative incidence (R)?
Definition
The proportion of a closed cohort at risk that presents the outcome within a period of time (total events/total people at risk)
Term
3 characteristics of cumulative incidence (R)
Definition
1. Unitless, 2. Represented as probability or %, 3. Numerator included in denominator
Term
What is cumulative survival (S)?
Definition
Compliment of cumulative incidence (S= 1-R)
Term
When do you use a life table vs a Kaplan Meier to estimate S?
Definition
Life table if exact time to event is unknown (assumes W is at t1/2), K-M if event time is known
Term
What is an incidence rate?
Definition
Number of new cases that occur in a specific period of time in a pop. at risk (#new cases/person-time)
Term
What is the relation between incidence and prevalence?
Definition
P= I x D
Term
What is an odds?
Definition
The probability of an event divided by the prob. of non-event (Odds= p/1-p)
Term
How can risk be estimated form an odds?
Definition
R= odds/(1+odds)
Term
What is the counterfactual model?
Definition
The risk experience an exposed individual would have had, had he not been exposed, all else being equal
Term
What is the immortal person-time?
Definition
Time before the study the individual had to survive in order to be included
Term
2 consequences of disregarding immortal person-time
Definition
1. Overestimation of total time at which the individual was at risk, 2. Underestimation of incidence rate for endpoint
Term
What is sensitivity?
Definition
Proportion of positive tests given the disease is truly present (a/a+c)
Term
What is specificity?
Definition
Proportion of negative tests given they are truly disease free (d/d+b)
Term
What is the PPV?
Definition
Given a positive test, proportion of individuals who actually have the disease
Term
What is the NPV?
Definition
Given a negative test, proportion that are not diseased
Term
What is the goal of cross-sectional studies?
Definition
To compare the prevalence of an outcome in exposed and unexposed
Term
What is the goal of cohort studies?
Definition
To compare the incidence of a disease in exposed an unexposed
Term
What is the goal of case-control studies?
Definition
To compare the odds of exposure in cases and controls (implies odds of disease in exposed and unexposed)
Term
What is the true causal effect?
Definition
Difference in the freq. of disease in a pop. with everyone exposed minus the freq. of disease in the same pop., during the same time, but unexposed
Term
What is the exchangeability/randomization assumption?
Definition
The mean in the unexposed group is exchangeable with the mean in the counterfactual exposed group
Term
How does stratification provide with exchangeability?
Definition
After stratification, exchangeability will hold between exposed and unexposed within the strata of the confounder
Term
What is a mediator?
Definition
A variable that is the causal pathway between exposure and outcome
Term
What is the difference between a full and a partial mediator?
Definition
Full: exposure has an indirect effect on outcome. Partial: exposure has a direct effect on outcome
Term
According to the Prentice criteria, what is a surrogate?
Definition
1. Must be correlated to outcome, 2. Must be a full mediator, 3. Must fully capture the effect of tx on an outcome
Term
What is a collider-stratification bias?
Definition
The opening of a backdoor pathway due to the adjustment of a mediator
Term
How many reference categories are there in a heterogeneity of effects table?
Definition
2 reference categories (within group comparisons)
Term
What are Cook's types of prediction models?
Definition
1.Diagnostic (accurate identification of current disease), 2. Prognostic (estimate risk of a future unknown event)
Term
What is calibration?
Definition
Ability to correctly estimate a disease state or risk of future event
Term
What is discrimination?
Definition
Ability to separate persons with and without the disease
Term
What does a ROC plot?
Definition
Sensitivity (Y axis) and 1-specificity (X axis)
Term
What is the C statistic or AUC?
Definition
The probability that a randomly selected individual with disease has a test result indicating greater suspicion of disease
Term
What is the hierarchy for model validation?
Definition
1. No validation, 2. Internal validation, 3. Prospective validation, 4. External validation
Term
Missing data can introduce what type of bias?
Definition
Selection bias
Term
What does missing completely at random assume?
Definition
the probability of missing data values does not depend on anything but chance
Term
What does missing at random assume?
Definition
the probability of missing data values may depend on observed values in the study, but not on missing values
Term
What does not missing at random assume?
Definition
the probability of missing data depends on the value of some missing data
Term
What are the causal response types?
Definition

1. No effect. Doomed

2. Exp Causal

3. Exp Protective

4. No effect. immune

Term
What are the potential outcomes?
Definition

D=1 outcome had they been exposed

D=0 outcome had they been un exposed

At best one is observed although both have a true value per person

Term
What is the ecological fallacy?
Definition
error of interpretation of statistical data; inferences about the nature of specific individuals are based solely upon aggregate statistics collected for the group to which those individuals belong.
Term
What is competing risk bias?
Definition
Type of seection bias where outcomes are mutually exclusive
Term
What is ascertaintment bias?
Definition
Type of selection bias where patients gathered  do not represent the cases originated in the population
Term
What is the Neyman bias?
Definition
Also called selective survival bias, type of selection bias where the exposure is determinant of or related to prognosis factors
Term
What is the healthy worker effect?
Definition
Lower mortality rate in employed individuals compared to the general population
Term
What is the Hawthorne effect?
Definition
Increase in the outcome under study in participants who are aware of being observed
Term
What is a protopathic bias?
Definition
When exposure is influenced by early (subclinical) stages of disease
Term
What is a contamination bias?
Definition
When intervention-type activities find their way to the control group
Term
Definition