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82 Cards in this Set

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Purpose of Clinical Trials:
To study the effectiveness of treatments in terms of causal relationships
Purpose of Cohort and Case Control studies:
To look at associations between risk factors and disease
Purpose of Diagnostic Testing studies:
To estimate accuracies and predictive values of diagnostic tests
What is an "independent variable"?
A CAUSE on which the EFFECT (dependent variable) is dependent.
In Evidence-Based-Medicine, what are we looking for evidence OF?
Causality - that a variable causes a changed effect.
Why aren't Koch's Postulates very good for determining causality?
Because they are so stringent that they don't allow for the multiple factors that can be associated with an effect.
What is the requirement of Temporality to provide evidence of causality?
The cause must precede the effect.
What is the requirement of Association to provide evidence of causality?
The cause and effect must be seen together more than expected by chance.
What other alternative explanations have to be considered?
Confounders
What is a Dose-Response pattern?
A relationship between cause and effect where the greater the cause, the greater effect.
8 Factors of Evidence for Causality:
1. Temporality
2. Strength of association
3. Dose Response
4. Consideration of alternative explanations
5. Consistency across studies
6. Biologic Plausability
7. Cessation of exposure
8. Specificity of association
2 Main categories of study design:
1. Experimental
2. Observational
How is an experimental study conducted?
Something is done to a group of patients and the impact is measured in comparison to a control group.
What are experimental studies generally referred to as?
Randomized controlled trials
How are observational studies conducted?
The researcher does not control what is done to the subjects, associations are just measured.
4 types of Observational Studies:
1. Ecologic
2. Cross-sectional
3. Cohort
4. Case Control
What is measured in an Ecologic study?
Associations between groups
Unit of analysis in an ecologic study:
Group
Disadvantage of ecologic studies:
Few conclusions can be made, and they are generally very poor and circumstantial.
Advantage of ecologic studies:
Allows you to look at larger geographic trends.
How is a Cross-sectional Study designed?
By starting with a sample and looking at what proportion is exposed, and what proportion is diseased.
What does a Cross-sectional study measure?
Prevalence
What is Prevalence?
The # of cases/Total population at a GIVEN POINT IN TIME.
What can/cannot be done with a Cross-sectional study?
CAN: build evidence for associations
CANNOT: establish causality
Why can't a cross-sectional study establish causality?
Because a CS study is weak
How is Measure of Association calculated from a cross-sectional study?
Prev of disease in exposed
--------------------------
Prev of disease in unexposed
For asthma, what would a prevalence ratio of 3 for heavy smokers compared to nonsmokers tell you?
That heavy smokers are 3 times more likely to have asthma than nonsmokers.
What is an example of a cross-sectional study?
A survey
Advantages of cross-sectional studies:
Quick, easy, and cheap
What does a Cohort Study analyse?
Risk and prognosis
How does a Cohort study start?
By chosing a group of patients based on clear criteria that are free from the outcome at the start.
After choosing a study population, how does a cohort study proceed?
By measuring the exposure or risk factor of interest.
How is it decided who gets exposed or not?
It isn't decided; the groups form naturally.
What happens after groups are differentiated into exposed vs nonexposed?
The researchers look forward in time and measure the incidence of the outcome in each group.
What is a key factor of evidence ofr causality that is demonstrated by cohort studies?
Temporality - because the investigators look into the future to look for the outcome.
What is a confounder?
A factor that is associated with (and effects) both variables but has not been accounted for.
2 ways that confounders can affect study results:
1. Cause spurious/false associations
2. Influence the size of the effect
What is the confounder in a cohort study that shows heavy drinkers are more likely to develop lung cancer?
Smoking - because heavy drinkers are more likely to be smokers.
What is the measure of outcome in a cohort study?
Incidence
What is Incidence?
The proportion of an at-risk population that DEVELOPS the factor of interest over a certain period of time.
What is Prevalence?
The proportion of an at-risk population that HAS the factor of interest at ANY GIVEN TIME.
For the flu, what is higher, incidence or prevalence?
Incidence - more people develop the flu over one year than have it at one given moment.
Why is incidence higher than prevalence for the flu?
Because it is such a short-lived illness.
What types of diseases have a higher prevalence than incidence?
Chronic, long-lasting diseases. Because the illness lasts so long that patients accumulate.
What is the measure of association in a Cohort study?
Relative Risk, aka Risk Ratio
Formula for Relative Risk:
Incidence of Outcome in Exposed
------------------------------
Incidence of Outcome in Unexposed
What does a RR=1 say?
There is no difference in exposed vs unexposed, hence no association with the outcome.
What does a RR>1 say?
Exposure causes the outcome.
What does a RR<1 say?
Exposure prevents the outcome.
Disadvantages of Cohort studies:
-Can be expensive
-Can be time-consuming
-Difficult to study the incidence of a rare disease
Why is it difficult to study the incidence of rare diseases?
Because you need such a huge sample population.
How do we practically study associations for rare occurrences?
Via Case-control studies.
What does a Case-control study start with?
Selecting 2 groups of patients:
1. Cases - already have the disease or outcome
2. Controls - don't have the disease or outcome
What do you want to be the same between cases and controls?
The population from which they came.
What happens after sample selection in a case-control study?
You look retrospectively to obtain information on risk factors, then see if there is an association with the outcome.
3 Advantages of a Case-control study compared to cohort:
-Quicker
-Cheaper
-Require no long follow-up period
3 Disadvantages of a Case-control study:
1. Can't establish temporality bc it's retrospective
2. Still have issues of confounding variables
3. Selection bias
What is the measure of association for a cohort study?
Incidence
What is the measure of association for a case-control study?
Odds ratio
Why can't we calculate incidence for a case-control study?
Because we START with the disease, so we can't determine what the risk of getting the disease is.
What do we do with the incidence calculated from a cohort study?
Calculate a Risk Ratio
What do we do with the odds that we calculate in a case-control study?
Calculate an Odds Ratio
How do you calculate Odds for a case-control study?
# with factor
-------------
# w/out factor
What is the probability of rolling a six with a die?
1/6 = 0.17
What are the odds of rolling a six with a die?
1/5 = .20
How does the Odds Ratio work as a measurement of association?
If the outcome is rare, the OR approaches the relative risk and is a good estimate of it.
A case-control study is:
always retrospective; the history of exposure is measured after the outcome is known.
Comparison groups in a case-control study are selected based on:
outcome
Case-control studies are useful when:
the outcome is rare
The measure of association for a case-control study is:
the Odds Ratio
What is the gold standard of study design?
Randomized Controlled Trials
How does an RCT start?
By selecting a group of subjects that meet clear inclusion criteria
What happens in an RCT after the study population is selected?
Allocation into 2 groups:
1. Experiment/intervention
2. Control
What is one of the strengths of the RCT?
Temporality can be established because you measure the outcome after causing the cause.
Why is it advantageous to "control" who gets the experimental intervention or not?
Because you can control alternative variables and keep them constant.
What is the measure of association in an RCT?
Incidence - calculated as relative risk.
How is Relative Risk calculated from an RCT?
Incidence of outcome in treated
-------------------------------
Incidence of outcome in control
What is meant by:
RR=1
RR>1
RR<1
RR=1 means no difference
RR>1 means treatment causes the outcome
RR<1 means treatment prevents the outcome
What stipulation must be met in order to use the relative risk ratio?
The outcome must be dichotomous - yes/no, better/worse, etc.
The RCT is the gold standard for:
Studies about treatment
3 Disadvantages of the RCT:
1. Expensive
2. Time-consuming
3. May not be ethically feasible
The RCT measures
Incidence/Risk ratio