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