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120 Cards in this Set

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3 "effects" that an RCT prospectively can measure:
1. Relative risk (dichotomous variables)
2. Mean differences (continuous variables)
3. Survival (time to event)
2 measures of statistical significance in an RCT:
-P-value
-95% confidence interval
Pvalue:
the probability that a result is due to chance
CI:
range of likely values
3 Uses of Survival Analysis:
-For time-to-event data
-To follow treatment of terminal illnesses
-When subjects are enrolled for different lengths of time.
Curve used for survival analysis:
Kaplan-Meier Survival curve
The Kaplan-Meier survival curve is based on:
the fact that probability of multiple independent events is the product of the probabilities of each event.
What test is used to determine whether 2 Kaplan-Meier survival curves are statistically significant? What will it generate?
A log-rank test - generates a P value
What are Cohort and Case-control studies used for?
Demonstrating associations between variables to show Etiology or Prognosis
Why would you want to use cohort or case-control studies vs RCT? (3 reasons)
-Less expensive
-More feasible
-Fewer ethical issues
A cohort study begins with:
A study population that is FREE OF DISEASE.
Who is excluded from a Cohort study population?
Any subjects that already have the outcome.
What is done in a Cohort study once the study population has been chosen?
Measurement and classification of the exposed and unexposed groups
Final step in a cohort study:
Measure the # of diseased vs undiseased in each group (exposed vs unexposed).
How is it decided who gets exposed and who doesn't in a cohort study?
Nature decides it.
Is there randomization, allocation concealment, or intention-to-treat in a cohort study?
NO - it is purely observational.
What is the temporality of a cohort study like?
Prospective or retrospective
What are 4 questions that give level 2 evidence and can be answered by a cohort study?
1. Questions of treatment
2. Questions of prevention
3. Questions of etiology
4. Questions of harm
What are 2 questions that give level 1 evidence and can be answered by a cohort study?
1. Questions of prognosis/ natural history
2. Questions about diagnosis
ways that you can define the outcome of a cohort study:
1. Incidence
2. Relative risk
3. Attributable risk (ARD)
4. Relative risk difference RRD
5. NNT
Definition of incidence:
The # of new cases of a disease that occur during a period of time in a population at risk for developing the disease.
2 Things required in order to be able to express incidence:
1. Numerator - the number of new cases
2. Denominator - the total population at risk for disease
EER:
Experimental event rate - the incidence in exposed.
CER:
Control event rate - the incidence in the controls
Relative Risk:
EER/CER
ARD
Absolute risk difference
EER - CER
RRD
Relative Risk Difference
EER-CER/CER
NNT
Number needed to treat
1/ARD
PECO
Population
Exposure
Comparison
Outcome
What is the significance of the PEcO format?
It should be included in the hypothesis that begins a cohort study.
How can you tell if a study's results are valid internally?
They should be free from bias and error.
What is Bias?
Systematic error that can give false findings.
What is Random Error?
Noise that reduces precision of measurements
What is a Confounder?
Another variable that influences the dependent variable and may actually be responsible for association or effect.
What is External Validity?
Generalizability
What factors in a study design contribute to generalizability?
Inclusion and exclusion criteria for sample selection.
Definition of Bias:
Any systematic error in a study design, conduct, or analysis, resulting in a mistaken estimate of an exposure's effect on the risk of disease.
What causes Selection bias?
Selecting or classifying exposed and unexposed subjects in such a way that an APPARENT association is seen, if if it's not true.
Common cause of selection bias:
Non-responders - people who decline to participate in a study.
What is a side-effect of non-responders other than selection bias?
Influence on generalizability - the responders may not represent the population of interest.
2 types of Misclassification of exposure:
-Random/nondifferential
-Systematic/Differential
What is Random/Nondifferential misclassification of exposure?
When errors in classification of exposed vs nonexposed to one study variable are independent of the level of other variables.
Exmple of Random/nondifferential misclassification of exposure:
-Data entry
What direction does bias tend toward in a random misclassific. of exposure?
Toward the null
What is Systematic/differential misclassification of exposure?
When errors in classification of one study variable depend on the status of other variables.
In what direction does bias tend in systematic/differential misclassification of exposure?
Bias is unpredictable - could result in a type I error.
How should the exposed/nonexpsd groups in a cohort study compare?
They should be similar in every way other than exposure.
How can you help to ensure that cohort groups are similar?
You can't intervene much - it's just an observational study so you can't randomize like in RCT.
3 ways to control confounding through study design:
1. Randomization
2. Selection
3. Matching
How does randomization limit confounding?
By distributing confounders evenly between groups
Can you randomize in a cohort or case control study?
No
How does Selection limit confounding?
By letting you study only those with or without the confounder present.
What is a disadvantage of Selection?
It limits generalizability
How is Matching done?
By selecting a control for each individual subject on the basis of confounders.
3 ways to control confounding through ANALYSIS:
1. Stratification
2. Adjustment
3. Multivariate analysis
What is stratification?
Seperation of study subjects into groups based on values of confounding variables.
What is adjustment?
Using statistics to control for a confounding variable's effect.
What does Multivariate analysis do?
Adjusts for or determines the influence of multiple variables at once.
Example of multivariate analysis:
Multiple regression
Major difference in how we deal with confounders in cohort studies versus RCT:
RCT - eliminates confounders
Cohort - control confounders
3 Techniques that identify independent variables that may be associated with the outcome in question:
1. Linear regression
2. Logistic regression
3. Proportional hazard regression
What is linear regression used for?
Continuous outcomes
What is Logistic regression used for?
Binary outcomes
What is a proportional hazard regression used for?
Time-to-event
2 types of linear regression:
-Linear regression for one continuous risk fx and outcome
-Multiple linear regression
What is Multiple linear regression used for?
Estimating the contributions of several continuous dependent variables on the outcome or to adjust for confounders.
What can Logistic Regression be used for?
-Multiple independent risk factors, which can be continuous or dichotomous
-One single dichotomous dependent variable
How are results from logistic regression expressed?
As individual odds ratios for each variable, holding the others constant.
What is another name for proportional hazards regression?
Cox regression
What is Cox/Ppl hazards regression related to?
Survival analysis - time to event data.
What is the Cox regression used for?
To adjust for contribution of multiple risk factors for time to event analyses.
So what is achieved by using cox regression?
Limit/control confounders in a time-to-event study.
3 types of Outcome Measurement Validity:
1. Face validity
2. Criterion-based validity
3. Predictive validity
What is face validity?
How much common-sense is in the measurement.
What is criterion-based validity?
How well a measurement agrees with other measurements of the same phenomenon (ie selfreport of phys activity vs pedometer)
What is predictive validity?
How well a measurement predicts an event.
What is POEM?
Patient Oriented Evidence that Matters
What is DOE?
Disease-Oriented Evidence
2 types of error:
-Random
-Systematic
What does Random error affect?
Precision or reliability
What does Systematic error affect?
Validity
How is random error commonly described?
-By standard deviation
How can random erro be minimized? (4 ways)
1. Create standards
2. Provide training
3. Refining instruments
4. Repeating measurements
3 types of systematic error:
1. Instrument bias
2. Investigator bias
3. Subject bias
Cause of instrument bias:
miscalibrated scale
What is investigator bias?
Differential interpretation of results based on knowledge of exposure
3 forms of Subject Bias:
1. Social desirability bias
2. Attribution bias
3. Hawthorne effect
What is the Hawthorne effect?
The change in behavior that may occur when subjects realize they are being observed.
How is Bias identified?
By comparison with a reference standard.
How is Systematic bias/error minimized? (3 ways)
1. Calibration
2. Blinding
3. Unobtrustive measurement
Major difference between Cohort and Case-control studies:
Cohort = prospective (mainly)
Case-control = retrospective
How does a Case-control study compare to others in terms of strength?
Not as strong as RCT or cohort
What level 3 evidence can be provided by a case-control study? (4 questions)
-Questions of treatment
-Questions of prevention
-Questions of etiology
-Questions of harm
What level 4 evidence can be provided by a case-control study? (1 question)
-Questions about diagnosis
What unique problem arises in case-control studies compared to cohort or RCT?
Selection bias because you're selecting the cases and controls that already have or don't have the outcome.
What is important about where you get your cases/controls from?
They should arise from the same population and be similar in every way except for outcome.
What does it do when you have >1 control per case?
Increases the power of the study.
What new source of measurement error arises in case-control studies?
Differential recall - because the subjects have to recall details of exposure.
How do you minimize confounders in a case-control study?
By matching cases and controls.
What analysis technique in a case-control study can minimize confounders?
Regression techniques
Why can't you calculate incidence in a case-control study?
Because you lack the denominator - the overall population at risk.
What is the outcome measure for a case-control study?
the Odds Ratio
What is an Odds Ratio?
the ratio of "likelihood of having been exposed" in diseased versus nondiseased.
What is Probability?
A # representing the likelihood that a specific event will occur
How is probability expressed?
As the ratio of the number of actual occurrences to the number of possible occurrences.
Examples of probability (2):
-Incidence
-Prevalence
What is Risk as defined and used properly?
A probability
What does it mean to say a woman's lifetime risk of breast cancer is 12%?
Out of 100 women ~12 will get it
What is Odds?
The probability of event divided by the probability of NO event
If a women's risk of breast cancer is 12% what are her odds of getting it?
12:88 (.13:1)
If there's an 80% risk of rain, what are the odds of rain?
80:20 = 4
Formula for Odds Ratio:
Odds of Exposure in cases
-------------------------
Odds of Exposure in controls
How does an Odds ratio compare to relative risk?
It approximates RR if the disease is rare.
How is an odds ratio interpreted?
Similar to relative risk
What does an OR=1 mean?
There is no association between the disease and risk factor.
What does an OR>1 mean?
There is a relationship of the risk factor to the disease - causative.
What does an OR<1 mean?
The risk factor is protective of disease.
What do you use if you want to look at several risk factors in a case control study?
Logistic regression
What does logistic regression do?
Controls for confounders, but does not eliminate them.
How are results from logistic regression expressed?
As an odds ratio