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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:

Pvalue
95% confidence interval 

Pvalue:

the probability that a result is due to chance


CI:

range of likely values


3 Uses of Survival Analysis:

For timetoevent data
To follow treatment of terminal illnesses When subjects are enrolled for different lengths of time. 

Curve used for survival analysis:

KaplanMeier Survival curve


The KaplanMeier 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 KaplanMeier survival curves are statistically significant? What will it generate?

A logrank test  generates a P value


What are Cohort and Casecontrol studies used for?

Demonstrating associations between variables to show Etiology or Prognosis


Why would you want to use cohort or casecontrol 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 intentiontotreat 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
EERCER/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:

Nonresponders  people who decline to participate in a study.


What is a sideeffect of nonresponders 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?

Timetoevent


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 timetoevent study.


3 types of Outcome Measurement Validity:

1. Face validity
2. Criterionbased validity 3. Predictive validity 

What is face validity?

How much commonsense is in the measurement.


What is criterionbased 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?

DiseaseOriented 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 Casecontrol studies:

Cohort = prospective (mainly)
Casecontrol = retrospective 

How does a Casecontrol study compare to others in terms of strength?

Not as strong as RCT or cohort


What level 3 evidence can be provided by a casecontrol study? (4 questions)

Questions of treatment
Questions of prevention Questions of etiology Questions of harm 

What level 4 evidence can be provided by a casecontrol study? (1 question)

Questions about diagnosis


What unique problem arises in casecontrol 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 casecontrol studies?

Differential recall  because the subjects have to recall details of exposure.


How do you minimize confounders in a casecontrol study?

By matching cases and controls.


What analysis technique in a casecontrol study can minimize confounders?

Regression techniques


Why can't you calculate incidence in a casecontrol study?

Because you lack the denominator  the overall population at risk.


What is the outcome measure for a casecontrol 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
