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